The Election Administration and Voting Survey, conducted by the Election Assistance Commission, collects data on this - the 2016 report states:
Over the past 20 years, the number of voters casting ballots using
by-mail absentee and inperson early voting has increased dramatically.
More states have adopted these modes of voting in an effort to make
voting more convenient, and voters are taking advantage of these new
options for voting. By-mail absentee voting allows individuals to
receive their ballot in the mail before the election and then mark
their ballot at their leisure. The voter typically puts his or her
marked ballot in an envelope and then mails it to his or her local
election office or places it in an absentee ballot drop-off box.
In-person early voting—which some states refer to as in-person
absentee voting—allows a person to have the same experience voting as
they would if they voted on Election Day. The voter typically votes on
the same type of voting equipment as they would on Election Day, but
does so during the weeks leading up to Election Day.
In 2016, 41.3
percent of all ballots were cast before Election Day. Of the total
turnout, 17.2 percent of ballots were cast using in-person early
voting and 23.7 percent were cast using by-mail absentee voting.1
1: The percentages are based on data reported by 50 states and territories. Alabama, Iowa, Utah, and
Vermont did not provide information about the number of citizens casting absentee ballots or voting at an
early voting center
According to the Federal Election Commission's report on the 2016 election, there were 136,669,276 votes cast, so this translates to ~23.5 million in-person early votes, ~32.4 million by-mail absentee votes, and a total of ~56.4 million votes being cast before Election Day.
As you note in your question, it's hard to draw too many conclusions based on comparisons between these figures due to the COVID-19 pandemic, however, clearly 94 million ballots represents a large increase.
