

Yeah, Proton runs outside of Steam in Heroic and Lutris. That’s basically what the umu project is about. I think it works in Bottles as well.
Almost everything Valve has done for Linux gaming is open source and will remain even if they go away and lock everything down tomorrow.
Proton is mostly a fork of Wine which has been used for decades to run Windows software on Linux. Valve didn’t do all the hard work by themselves.