The free version offers limited features, including 5 AI audio separations per month but, if that's all you need, why not? Anyway, you can find out how good it is for free,
Moises plans and billing Fadr, likewise, has free and paid plans.
Most stem separators are built from
Spleeter, an open-sourced freeware platform. The downside is that you must write your own GUI.
Stagecraft Software is shareware in that you can download and use their tools for free but they would like you to pay within 30 days — Holy 1986! In any case, no monthlies or subscription involved and everything is done in the box (your Mac or PC) with no usage limits. Here's a link to their Spleeter based
Simple Stems for $20. I use this on occasion.
I haven't found any stem separator to be as good as advertised and I own quite a few including high priced products from iZotope and Yamaha/Steinberg.