To keep it simple one could say userspace is what the average user faces. Eg the audio engine, the desktop shell, the window tiler,… So basically the things that make your apps and make them work.

Windows 11 has the ability to natively run XP software iirc because they don’t break the underlying systems, they rather extend them.

macOS in the other hand changes stuff all the time and if a developer doesn’t catch up with that, the next macOS release will break the software.