StemDeck
A free open-source desktop app that splits songs into separate instrument tracks on your own computer, with no upload and no account.
StemDeck is an open-source application for Windows, macOS and Linux that takes a finished song and separates it into as many as six parts: vocals, drums, bass, guitar, piano and everything else. You can then mute, solo and rebalance those parts in a mixer that looks like ordinary recording software, loop a section, and export what you want.
The point of it is where the work happens. Commercial stem splitters run in the cloud behind an account and a subscription, which means uploading whatever you are working on. StemDeck runs the separation model, an open-source system called Demucs, entirely on your own machine, using an Nvidia or Apple Silicon graphics chip if you have one and the processor if you do not. The project is still in an early alpha stage. Separating a track also does not grant you any rights to it, so practice and transcription are fine while publishing a remix still needs permission.