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?
There are techniques to insulate the codebase. For example, you can have one person read the actual leaked code, explain the data structures and algorithms at a high level to a developer, then have the developer implement that logic themselves based only on what they understood from the explanation. I believe this is known as clean-room reverse engineering.