This post is not comprehensive (and can't be given the space limitations of answer at Politics.SE), but notes some selected notable changes in voting systems and political systems (which change what is even being voted for in addition to electoral systems) in recent times.
It is a convenience sample based upon the examples that came to mind first. It details some of the changes made in 25 specific countries, and also notes two major waves of changes in Europe (post-WWII and post-Soviet) in addition.
Political Systems Rooted In The British System
united-states
Federal reforms
The United States replaced a system where U.S. Senators were appointed by state legislatures with one where they were directly elected in 1913 with the 17th Amendment to the United States Constitution.
The Voting Right Act of 1965 in the U.S. significantly reformed the redistricting process and eventually, it was used to prohibit some local government election methods found only in the South, such as the practice of having only a single elected official in some counties that had complete executive and legislative branchy authority over county government affairs (a practice designed to insure no African-Americans were elected to county office in counties with a narrow white majority in the voting population).
A series of U.S. Supreme Court cases in the 1960s mandated a one-man, one-vote rule for state legislatures and local legislative bodies requiring electoral districts to have approximately equal populations. Prior to this, many states had one house of their legislature elected based upon county lines in a way parallel to the U.S. Senate, effectively over representing rural areas.
The U.S. discontinued the practice of allowing some states to have multi-member districts for U.S. House seats by legislation effective in the 1972 election at 2 U.S.C. § 2c. (Source) The practice of multi-member house district had been used to prevent blacks in the American South from being able to elect representatives.
Not long after the U.S. Civil War, the practice of having junior officers in the U.S. Army elected by their subordinates was discontinued.
State level reforms
There were several such measures in the U.S. in 2024 at the state level (the U.S. doe not have national referendums). AZ prohibited jungle primaries. AK ended its current use of instant runoff voting. CO rejected a measure to add jungle primaries and instant runoff voting.
California adopted a jungle primary system in 2010; Washington State did so in 2004 effective in 2008; Alaska did so in 2020. (Source)
Maine adopted instant runoff voting in 2016. Alaska did so in 2020 and repealed that change in 2024.
Nebraska revamped its state legislature in 1934 to be unicameral and non-partisan:
When Nebraska became a state in 1867, its legislature consisted of two
houses: a House of Representatives and a Senate. For years, U.S.
Senator George Norris (Senator 1913–1943) and other Nebraskans
encouraged the idea of a unicameral legislature and demanded the issue
be decided in a referendum. Norris argued:
"The constitutions of our various states are built upon the idea that
there is but one class. If this be true, there is no sense or reason
in having the same thing done twice, especially if it is to be done by
two bodies of men elected in the same way and having the same
jurisdiction."
Unicameral supporters also argued that a bicameral legislature had a
significant undemocratic feature in the committees that reconciled
House and Senate legislation. Votes in these committees were
secretive, and would sometimes add provisions to bills that neither
house had approved. Nebraska's unicameral legislature today has rules
that bills can contain only one subject, and must be given at least
five days of consideration. In 1934, due in part to the budgetary
pressure of the Great Depression, Nebraska citizens ran a state
initiative to vote on a constitutional amendment creating a unicameral
legislature, which was approved, which, in effect, abolished the House
of Representatives (the lower house).]
Secret ballots were adopted gradually, on a state by state basis, at the state level in the U.S. over the time period from 1884 to 1950.
united-kingdom
In addition to the 2011 election referenced in the question, there was a major overhaul of the House of Lords in 1999 (by legislation rather than referrendum). Prior to 1999, all hereditary lords could vote in the House of Lords (more than 700 of them). Since 1999, the Lords vote among themselves for about 90 Lords entitled to vote in the chamber (in addition to some individuals such as bishops, who serve, ex officio).
The U.K. adopted the secret ballot from 1870-1872, but "the UK uses numbered ballots to allow courts to intervene, under rare circumstances, to identify which candidate voters voted for." So, its ballots aren't truly secret even today.
canada
The province of British Columbia changed to an instant runoff voting system in 1952 for provincial legislative elections, and then to multi-member districts for some provincial legislative seats in 1990. According to the same source:
IRV was also used for provincial elections in Alberta (1926–1955 in
rural districts), and Manitoba (1927–1953) outside Winnipeg (excluding
St. Boniface in 1949 and 1953). IRV was also used in provincial
by-elections in these two provinces between 1924 and 1955.
Measures to switch to a proportional system in British Columbia failed in 2005, 2009 and 2018.
Reforms to the means by which Canada's Senate is selected, which is appointed and much like the House of Lords has little real power, have been periodically proposed, but have never gotten very far.
australia
Australia adopted a single transferrable vote system in 1918.
Australia instituted compulsory voting between 1915 and 1984 across its various jurisdictions. At a Federal level, it was instituted for all British Subjects in 1924 and extended to indigenous Australians in 1984.
Queensland abolished its upper house in 1922; however, this was not an elected body with members appointed by the Governor on the advice of the government of the day. Queensland remains the only unicameral parliament in the Commonwealth of Australia.
Australia was also one of the modern adopters of the secret ballot:
In Australia, secret balloting appears to have been first implemented
in Tasmania on 7 February 1856.
Until the original Tasmanian Electoral Act 1856 was "re-discovered"
recently, credit for the first implementation of the secret ballot
often went to Victoria, where the former mayor of Melbourne William
Nicholson pioneered it,9 and simultaneously South Australia.10
Victoria enacted legislation for secret ballots on 19 March 1856,11
and South Australian Electoral Commissioner William Boothby generally
gets credit for creating the system finally enacted into law in South
Australia on 2 April of that same year (a fortnight later). The other
British colonies in Australia followed: New South Wales (1858),
Queensland (1859), and Western Australia (1877).
State electoral laws, including the secret ballot, applied for the
first election of the Australian parliament in 1901, and the system
has continued to be a feature of federal elections and referendums.
The Commonwealth Electoral Act 1918 does not explicitly set out the
secret ballot, but a reading of sections 206, 207, 325, and 327 of the
Act would imply its assumption. Sections 323 and 226(4), however,
apply the principle of a secret ballot to polling staff and would also
support the assumption.
new-zealand
New Zealand had changed its voting system:
Almost all parliamentary elections between 1853 and 1996 were held
under the first past the post (FPP) electoral system. Under FPP the
candidate in a given electorate (district) that received the most
votes was elected to the House of Representatives. The only deviation
from the FPP system during this time occurred in the 1908 and 1911
elections when a second-ballot system was used; the second-ballot
legislation was repealed in 1913. The elections since 1935 have been
dominated by two political parties, National and Labour.
Public criticism of the FPP system began in the 1950s and intensified
after Labour lost elections in 1978 and 1981 despite having more
overall votes than National. An indicative (non-binding) referendum to
change the voting system was held in 1992, which led to a binding
referendum during the 1993 election.13 As a result, New Zealand has
used the mixed-member proportional (MMP) system since 1996. Under MMP,
each member of Parliament is either directly elected by voters in a
single-member district via FPP or appointed from their party's list.
Parliament normally has 120 seats, though some elections have resulted
in overhang, as is currently the case (as of December 2023). In the
first eight elections under MMP, from 1996 to 2017, no party won a
majority of seats.
Seven electorates are reserved for MPs elected on a separate Māori
roll. However, Māori may choose to vote in and to run for the
non-reserved electorates and the party list (since 1996), and as a
result, many have now entered Parliament outside of the reserved
seats.
New Zealand implemented secret voting in 1870.
south-africa
When apartheid fell, South Africa adopted an interim constitutional system in 1994 and a formal new constitution in 1997. These reforms greatly changed the system from one with separate ethnically based legislative houses with unequal powers to a unitarian system without ethnic divisions. South Africa's new upper house is structurally similar to the pre-17th Amendment U.S. Senate, but like the Canadian Senate and the United Kingdom's House of Lords, it has little power.
Countries With Large Islamic Populations
egypt
Egypt's electoral system originated in 1952, which turned out in practice to be basically a Soviet style system. There was a major reform of the system in 2011. This was interrupted with a 2013 military coup, followed by a new system in 2014 and another in 2019.
iraq
Iraq transitions from basically a one party system to a new electoral system in 2005, in the wake of the Iraq War.
afghanistan
Afghanistan became a republic (it was previously a monarchy) in 1973. It changed that system to a Soviet style Republic in 1978. In 1992, new leaders were elected in a period where the country was torn by chaos and the Taliban starting in 1996 started to control more and more territory, de facto, until the U.S. invasion which appointed a new interim leader in 2001. In 2002, Afghanistan adopted a new political system which remained in place until the regime collapsed with the final withdrawal of U.S. troops in 2021, with very different Taliban rule (which had been forming a shadow government in the regios it controlled since 2009) coming in its place.
lebanon
Lebanon's political system was majorly reformed in 1943:
The 1943 National Pact, an unwritten agreement that established the
political foundations of modern Lebanon, allocated political power on
an essentially confessional system based on the 1932 census.
The Parliament is elected by adult suffrage based on a system of majority or "winner-take-all" for the various confessional groups.
Democratic elections were suspended in Lebanon from 1975-1990. Elections were held in 1992. Major reforms and a new constitution were adopted in 2004 in anticipation of the withdrawal of Syrian troops.
Elections which are supposed to be conducted every four years were also suspended from the 2009 election until the 2018 election, skipping one election cycle entirely and holding the next one a year later.
turkey
Turkey reformed its political and electoral system in 2017.
indonesia
Indonesia adopted major reforms to its electoral system in a series of four constitutional amendments from 1999-2002, in the wake of the fall of Suharto who had effectively ruled the country as a dictator for decades.
Other Countries In East Asia, Southeast Asia, and Oceania
south-korea
In South Korea:
Since 1948, the constitution has undergone five major revisions, each
signifying a new republic. The current Sixth Republic began with the
last major constitutional revision that took effect in 1988. From its
founding until the June Democratic Struggle [ending in 1987], the
South Korean political system operated under a military authoritarian
regime, with the freedom of assembly, association, expression, press
and religion as well as civil society activism being tightly
restricted. During that period, there were no freely elected national
leaders, political opposition is suppressed, dissent was not permitted
and civil rights were curtailed.
Of course, the Korean War from 1950-1953, with a North Korean invasion that reached much of the South for a brief period, also impacted its political development.
Prior to the current 1988 constitution:
Since the 1972 implementation of the Yushin Constitution by
then-president Park Chung-hee, South Korean presidents were elected
indirectly by the National Conference for Unification, an electoral
college. This system persisted even after Park was assassinated and
then replaced by Choi Kyu-hah, who was himself replaced within months
by Chun after the Coup d'état of December Twelfth. Since the college
was generally hand picked by the regime itself, it did not represent
any sort of democratic check on presidential power.
The 1980 constitution instituted a seven year term limit for the President, which was, in turn, the trigger for the 1987 popular uprising that led to the 1988 constitution in place today.
peoples-republic-of-china
Mainland China abolished its short-lived Western style political system, established in 1906, in favor of the Maoist one-party state of the People's Republic of China in 1949.
China had non-partisan democratic elections for village chiefs from 1978-2018. A one-party system similar to those for other offices was phased in from a period not long before 2018 through 2023.
taiwan
Taiwan was ruled by the absolute monarch of China from 1683-1895, and then by the Japanese emperor from 1895-1945.
The old regime of the Republic of China retreated to the island of Formosa (i.e. Taiwan) and adopted in a new constitution in 1947, which was less democratic by modern standards than the system in place during the Republic of China. But it was irrelevant because from March 1947 until 1987, Taiwan was in a state of martial law.
Its electoral rules were reformed to establish a genuine democracy in 1991.
fiji
Fiji adopted instant runoff voting in 1998 and ended it in 2013.
papua-new-guinea
Papua New Guinea changed to an instant runoff voting system in 2002.
Continental Western Europe
Most countries in continental Europe adopted some form of new electoral system as post-Nazi constitutions were adopted after WWII.
germany
Germany had a unicameral, party-list system before WWII, but changed it after WWII to have a minimum threshold of 5% for a party to gain representation.
italy
Italy changed its electoral system in the 1990s:
Italy's dramatic self-renewal transformed the political landscape
between 1992 and 1997. Scandal investigations touched thousands of
politicians, administrators and businessmen; the shift from a
proportional to the scorporo system (with the requirement to obtain a
minimum of 4% of the national vote to obtain representation) also
altered the political landscape.
The change in Italy also created, in effect, a combined primary election and general election, giving voters a say in which candidate from a party list is elected at the same time that a party is chosen in the general election.
france
France radically overhauled its political system three times during the first French Republic that commenced in 1792, before reverting to an Empire, then adopted a new electoral system for the Second Republic from 1848 to 1852 before reverting to monarchy, and then adopted a new electoral system in the Third Republic in 1870. It adopted a new post-Vichy electoral system in 1946, and reformed its electoral politics again in the Fifth Republic in 1958. De Gaulle was first elected president by an electoral college in 1958 and then in 1962 called a referendum, which provided for a stronger directly elected President.
The French two round system with a second round election if no candidate receives a majority in the first round was adopted in 1832 by royal decree. France first started to use secret ballots due to an 1831 statute.
Parliamentary elections were converted to a party list proportional representation system in France starting with the 1986 election. But this system was abandoned in the next parliamentary election in France.
A more detailed and precise account for France, which has been particularly active in experimenting with different electoral systems over the course of its history, is provided in another answer.
spain
Spain
From 1833 until 1939 Spain almost continually had a parliamentary
system with a written constitution. Except during the First Republic
(1873–74), the Second Republic (1931–36), and the Spanish Civil War
(1936–39), Spain also always had a monarchy.
Spain adopted significant political reforms in its electoral system when it adopted its current constitution in 1978. Since then, there have been multiple efforts (in a rather complex series of events) to change the legal status of the Basque and Catalonian regions.
Eastern (i.e. Soviet influenced) Europe and Central Asia
All of the formerly Soviet Republics, all of the Russian Republics, the successor states to Yugoslavia, and the WARSAW Pact countries all changed their electoral systems in connection with the dissolution of the Soviet Union in 1989, and then followed in short order by the break up of Yugoslavia.
bosnia
Bosnia used a single transferrable vote system on a one time basis in its 2000 election.
poland
In 1989, Poland had its first multi-candidate election in more than 40 years after decades of Soviet-style one party elections, in the wake of the collapse of the Soviet Union:
In an attempt to take control of the situation, the contemporary
government gave de facto recognition to the Solidarity union, and
Interior Minister Czesław Kiszczak began talks with Solidarity's
leader Lech Wałęsa on August 31. These talks broke down in October,
but a new series of negotiations, the "round table" talks, began in
February 1989. These talks produced an agreement in April for partly
open parliamentary elections. The June election produced a Sejm (lower
house), in which one-third of the seats went to the communist party
and one-third went to the two parties which had hitherto been their
coalition partners. The remaining one-third of the seats in the Sejm
and all those in the Senate were freely contested; the majority of
these were by candidates supported by Solidarity. The failure of the
communist party at the polls produced a political crisis. The
round-table agreement called for a communist president, and on July
19, the National Assembly, with the support of a number of Solidarity
deputies, elected General Wojciech Jaruzelski to that office. However,
two attempts by the communists to form governments failed.
On August 19, President Jaruzelski asked journalist/Solidarity
activist Tadeusz Mazowiecki to form a government; on September 12, the
Sejm voted approval of Prime Minister Mazowiecki and his cabinet. For
the first time in more than 40 years, Poland had a government led by
non-communists.
In December 1989, the Sejm approved the government's reform program
. . . amended the constitution to eliminate references to the "leading
role" of the Communist Party, and renamed the country the "Republic of
Poland". The communist Polish United Workers' Party dissolved itself
in January 1990, creating in its place a new party, Social Democracy
of the Republic of Poland. . . . The May 1990 local elections were
entirely free. Candidates supported by Solidarity's Citizens'
Committees won most of the elections they contested, although voter
turnout was only a little over 40%. The cabinet was reshuffled in July
1990; the national defence and interior affairs ministers (hold-overs
from the previous communist government) were among those replaced.
In October 1990, the constitution was amended to curtail the term of
President Jaruzelski. In December, Lech Wałęsa became the first
popularly elected President of Poland.
Poland's Constitution was amended again in 1997:
The 1997 Constitution and the reformed administrative division of 1999
required a revision of the electoral system, which was passed in April
2001. The most important changes included:
the final liquidation of the party list (previously, some of the members of parliament were elected from a party list, based on
nationwide voter support, rather than from local constituencies),
modification of the method of allocating seats to the Sainte-Laguë method, which gave less premium to large parties. The latter change
was reverted to the d'Hondt method in 2002.
President Duda's government (since 2015) has made some controversial reforms characterized as democratic backsliding, but these have been largely to restrictions on the freedom of speech and in judicial branch reforms, rather than in electoral system reforms.
Some of the reforms were driven by Poland's ultimately successful bid to join the NATO and the European Union.
Latin America
mexico
Mexico adopted a constitution similar in model to that of the United States in 1917. From 1929 to 1987, this evolved into a dominant party system in which other political parties were allowed to compete and sometimes won a few elective offices, but the Partido Nacional Revolucionario (later the Institutional Revolutionary Party, known as the PRI), was dominant and remained continuously in control.
In 1951, the PRI passed legislation that made it harder to form new political parties. In 1963, under President Adolfo López Mateos, the PRI decided to reform electoral law to allow parties other than the PRI to have representation in Congress: if opposition parties obtained at least 2.5% of the national vote they would receive five deputies. In 1977, PRI President Echeverría oversaw an overhaul of electoral reform which lowered the number of members needed to officially register a new political party, and increased the number of seats that would be chosen through proportional representation.
In 1990, however, after a two party system emerged in 1988, Mexico as part of a series of electoral reforms adopted greater proportional representation in legislative elections. As a result, now, 300 of the deputies in the lower house of the legislature are elected by majority vote from single-member districts in their respective states, while the remaining 200 are elected via proportional representation party lists. In the upper house of the legislature (the Senate), each state elects three senators – two of these are allocated through a relative majority and the third seat is given according to the first minority principle, meaning it is given to the party that earned the second largest number of votes, and the remaining 32 seats are appointed through a proportional representation system according to the voter rolls at a national level, and the natural quotient and largest remainder electoral formulas are used.
Several smaller electoral reforms were adopted after 1990, and in 2002, 30% of candidates for all political parties in the general election were required to be women.
venezuela
The politics of Venezuela followed a typical libral democratic model
starting in 1958 after the fall of dictator Marcos Pérez Jiménez until
the 1990s. This period, known as the "Fourth Republic", is marked by
the development of the 1958 Punto Fijo Pact between the major parties.
By the end of the 1990s, however, the now two-party system's
credibility was almost nonexistent. . . . By 1998, support for
Democratic Action and COPEI had fallen still further, and Hugo Chávez,
a political outsider, won the 1998 election.
Chávez launched what he called the "Bolivarian Revolution" and
fulfilled an election promise by calling a Constituent Assembly in
1999, which drafted the new Constitution of Venezuela. Chávez was
granted executive power by the National Assembly to rule by decree
multiple times throughout his tenure, passing hundreds of laws. Chávez
ruled Venezuela by decree in 2000, 2001, 2004, 2005, 2006, 2007, 2008,
2010, 2011. and 2012. . . . Hugo Chávez, the central figure of the
Venezuelan political landscape since his election to the presidency in
1998 as a political outsider, died in office in early 2013 after a
long struggle with cancer. Nearing his death, Chávez expressed his
intention that his vice president would succeed him. Chavez was
succeeded by Nicolás Maduro, his vice president, initially as interim
President, before he narrowly won the 2013 Venezuelan presidential
election.
The 1999 Constitution was ratified in an election that year. It created a weaker unicameral legislature (it was previously bicameral with a combination of direct representation and proportional representation under the 1961 Constitution then in force) and a stronger Presidency with longer terms, and created a power to recall the President.
Further wide-ranging amendments were rejected narrowly in a referendum in 2007. Term limited were abolished in 2009 in a narrowly approved referendum.
