The feature was removed with iOS 15.4. With iOS 15.4, Apple disabled the ability to change the playback speed in third-party apps using the Apple Music API ...

Ideally, Apple will comment on this situation sooner rather than later and explain what’s going on. It’s possible that an inadvertent change to the Apple Music API is impacting Cider.

With iOS 15.4, Apple removed an API used by third-party Apple Music players that let users change the playback speed of songs.