I have dug around and can’t find it. I use it a lot and hope it isn’t gone forever
It hasn’t been removed! I just moved it, and haven’t documented it yet – still working on the updated Help pages.
You’ve probably already opted into the feature, but for anyone else reading this, the first step is to go to Settings → Experiments → Backing track recorder. Once enabled, you’ll have a new button in the volume mixer panel:
Tap that and then enable “Record next playback” and the rest works the same as before.