The feature to change the playback rate only applies to content owned by the user, such as songs purchased on the iTunes Store. So, help me understand: if I record tracks of my own music, composed and played by me, recorded using software I wrote, I can't change the playback rate?

The process changed in Xcode 13, and there are tips in the release notes. https://developer.apple.com/documentation/xcode-release-notes/xcode-13-release-notes

What's the crashing thread's backtrace?

Anyone who knows can't tell you. Further, just because something is planned doesn't mean the feature makes it in the final; also, some things make it in that even Apple engineers not directly working on it know about. Source: am former Apple engineer.