From Hardlimit
They do work without steam. Steam has just streamlined the process.
And of course you’d need a compatibility layer (e.g. wine/proton). But those aren’t dependant on Valve. Even though they’ve definetly benefited from their involvement.
You can check community reports on http://protondb.com
It’s an estimation since proton has made such giant leaps. When a new game releases, you can assume that it runs on linux.