https://docs.vrchat.com/v2020.3.2/docs/latest-release

>Full Body Tracking has had some changes! This is necessary due to an upgrade of the system we use for FBT. Don't worry, you shouldn't need to make any changes to your rigs (assuming you're not using any "rig hacks"). There's a lot going on here-- if you're a FBT user, you'll want to read through this full list!

major fbt change in here