Updating the thread... Using an app group works just fine for sharing preferences but that shouldn't be required between an app and included appex. If I can make a reduced test case I'll file a feedback on it.

Thanks edford. The issue is that I've found very few developers that have updated the plugins on the store as most missed this in the release notes. I've sent messages to the ones that I use that never updated, but none have replied. The situation as it stands is that most of the Xcode plugins in the MAS don't work with Xcode 12 on BS. It's not an ideal look. Thanks again, Josh

It's part of the Authentication Services - https://developer.apple.com/documentation/authenticationservices/ascredentialproviderviewcontroller. It's been around on iOS since 12, but it is new to the Mac on macOS BS.