So I’m talking about playing previously Windows-only games on Linux, e.g. via proton.
I don’t know about the libraries etc that are used - is it possible for Microsoft to use some legal voodoo, for example, to suddenly end it all, and make the use of their libraries illegal (if they belong to Microsoft in the first place)?
Or could there be other ways of interference?
Sure, but Microsoft has since contributed a lot to Linux and other open source projects. That’s not me saying “oh they’ve changed!”, that’s me saying they’ve made it significantly harder on themselves to bring legal action against because they’ve publicly endorsed and supported the project for so long.
Whatever legal arguments they tried in the past that failed are even weaker now.