People have also been doing audio source separation effectively for years without neural networks.
For example if you Google then FASST is one of the ones that come up, but it's a whole framework and in order to use it you'd have to learn the research yourself; much of these software is not geared for end users.
This is how I play guitar to my favorite songs on my computer.
Alternatively (in theory, anyways,) you can do a spectral analysis, create a selection/range of frequencies to invert, and that can usually handle isolating individual instruments regardless of stereo isolation as long as the other instrument octave ranges do not overlap with the one you're isolating. I'd love a music player that could do that.