If you’re a Java team, the geometry and kinematics classes don’t have any other dependencies, so you can just compile them separately. (WPILibC uses C++17 these days, so that’s a bit trickier.)
If you just want to test out the new kinematics from a library perspective, then clone the WPILib repository and build gradle manually via the command line:
Then add those jars as dependencies to your project. I just replaced a lot of our scattered kinematics with WPILib 2020’s stuff today.
You do NOT need the RIO image to test out the kinematics, but I’d wait before getting into anything that uses the HAL directly.