- The Prohibition was implemented and then ended, i.e. the state gave up its power to ban alcohol.

- The Bill of Rights itself post-dates the founding of the USA. Those amendments were limiting the power of the state!

- Income tax rates were once much higher than they are today. Of course you could argue that this isn't a reduction of its power given that once upon a time there was no income tax. But it has nonetheless fallen from its once great heights.

- The federal government gave up its power to regulate abortion quite recently.

- In the 60s (or 70s I forget) the US government deregulated the airline industry and has never gone back.

- The War Powers veto. One could argue that it's not been effective because POTUSes have ignored it, but in theory Congress took away the ability for Presidents to declare war.