Subject to terms and conditions. Come on now what kinda question is that?
My understanding regarding terms and conditions for subscription based stuff is that advanced payment conditions still has to be met, it's just month by month doesn't matter. So Netflix for instance can change whatever they want because people can just cancel. But I am probably completely wrong, I just misinterpreted it.
The Game Pass terms and conditions state,
"Service, features, and requirements may change or be retired."
So they probably could without much legal trouble. T&Cs aren't iron clad but unless Microsoft specifically stated, "You will be able to play every single game from every single Microsoft owned developer on day 1 with your subscription forever.", I don't think they'd be in legal trouble.
Yeah fair enough. Still seems hell messy though.