To prevent third-party applications utilising the Apple Music API and associated content from altering playing speed, Apple removed this feature in iOS 15.4. As in iOS 15.5, however, the functionality has been brought back.
Developer Joe Kun announced the news on the Apple Developer Forums, saying that Apple re-evaluated their “previous decision to disable changing the playback rate for subscription content from Apple Music in third-party applications” and concluded they “could safely enable that functionality again, just like before the release of iOS 15.4.”
Thus, the problem has been resolved in iOS 15.5 beta 4, according to Kun.
There was no explanation from Apple as to why third-party applications were unable to modify playback speed, but a post by Kun makes it clear that an issue has been resolved.
iOS 15.5 is still in beta, so users on the stable channel will have to wait until they can start altering the playing speed in third-party applications using the Apple Music API.