The 10th amendment is also relevant in recent discussions of police reform. Basically, because of the 10th amendment, congress can't just change how the police work because that's up to the states.
The Constitution establishes a “system of dual sovereignty between the States and the Federal Government.” Under the Tenth Amendment, “[t]he powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.” Thus, states generally have broad authorityto enact legislation, including to regulate the states and its localities’ law enforcement approaches. In contrast, Congress may only enact legislation under a specific power that is enumeratedin the Constitution and cannot use its power to intrude impermissibly on the sovereign powers of thestates. In this vein, the Supreme Court has recognized that there are certain subjects that are largely of a local concern where states “historically have been sovereign,”such as issuesrelated to the family, crime, and education.
source: Congressional Research Service legal sidebar 10487 (pdf warning)
Below is an example of this, relating to use of chokeholds.
The Congressional Research Services put together a summary (updated yesterday) of police reform bills by senate Republicans and Democrats. See "Congress and Police Reform: Current Law and Recent Proposals" (pdf warning).
The bill sponsored by the Congressional Black Caucus would limit funding to local/state police unless local laws or policies are in place to ban chokehold use.
Section 363 would require states that receive Byrne JAG funding to enact laws banning the use of chokeholds by law enforcement officers.
The Republican bill bans chokehold use by federal officers ("except when deadly force is authorized"). Which doesn't apply to state police agencies.
Section 105 would require recipients of certain federal funding to develop law enforcement agency policies “prohibit[ing]the use of chokeholds except when deadly force is authorized.” This section would also require the Attorney General to develop such a policy at the federal level.