I must preface this by saying that I'm not a game dev, but the Steam Runtime SDK has been offering stable versioned targets for linux for close to a decade now. Porting to one of them shouldn't be more difficult than porting to any other console specific SDK.
How the turntables…
If it doesn't run and you are a commercial client, Microsoft will implement a compatibility hack for you in the latest windows code so your thing from 1995 will work.
There is no parallel to this in the tech world so far. Linux gets around this by requiring you to recompile things, but recompiling old code along with old compilers and old libraries and all their dependencies is a nightmare.