This is a project I've been playing with for the past week. It's basically just an LSPosed module that re-enables the unused Sing slider in the Android app, and hooks it to an actual Neural Network for Stem separation. It runs incredibly well on recent Snapdragon devices since it can leverage its dedicated NPU through their official library: the most lightweight model present in the app (MDX-Net 1-9703) peaked at a processing speed of 60x Real-Time on my Snapdragon 8 gen 3; but basic support for GPU and CPU acceleration is provided for other devices as well via MNN.
As a bonus, the official Apple Vi-NNet Neural Model is available in the app purely for comparison purposes for the time being, although it runs very slowly.
If anyone is interested in trying it out, builds are available on GitHub: https://github.com/22cms/sing-for-android. All you'll have to do is install the app patch your Apple Music (preferably 7.0.0-beta) with LSPatch in integrated mode, or just use LSPosed/Vector if you are rooted.
I'm not sure if I will actively support this project in the future, as my main motive for making this was just to prove a point: Apple could very well port many AI-powered features to Android, they just don't wanna have to deal with it; but we'll see!