Well... I'm floored. I did not expect a further acknowledgment let alone a reversal on this. A bit late for me unfortunately as I've already removed this functionality and moved product and marketing in a completely different direction. Still, I'm glad the functionality is reinstated and I hope it gets ongoing support and updates. @JoeKun - Thank you for continuing to investigate and advocate for the third party developers on this issue.

My original bug report is in the same state as yours. Radar/Feedback has long been a blackhole so I would not expect a timely response there. I have AVAsset related bugs that have been "open" for 3-4 years. You could try a code-level support ticket but I would not expect to get a fundamentally different answer or even any further clarification to this issue. As for an explanation from Apple I think we have our answer above from @JoeKun. This was a 'bug' that is now 'fixed'. Given how buggy and unreliable MPMusicPlayerController has always been this seems a likely explanation.

The restriction here appears to be that tracks protected by DRM, such as those associated with Apple Music, cannot have their playback rate adjusted via MPMediaPlayer. Theoretically all other tracks can be playback rate adjusted either via MPMediaPlayer or via more robust APIs such AVAudioEngine + AVAudioUnitTimePitch.

Feedback Assistant is really the only option for both developers and end-users. And we all know how much good that will do. I did file a feedback on this when I made the original post. I have, of course, heard nothing and it remains 'open'. I think the chances of Apple revisiting this "decision" are very slim. The most likely scenario is that this was simply a "bug" all along and it is now fixed so far as they are concerned. Given the abysmal state of the MediaPlayer APIs, and their documentation, this should be no surprise to any of the developers here. Unfortunately this is simply an issue that fell through the cracks for many years and is not a large enough issue for Apple to be concerned with despite it being their mistake. Frankly I was surprised to get a response at all and I will be even more surprised should we get another.

Thank you, my search did not surface that previous post. I will begin following that post for additional updates.