That very much depends on the game. Multiplayer games are at the highest risk of becoming unplayable. Most single player game should in theory survive the death

That very much depends on the game. Multiplayer games are at the highest risk of becoming unplayable. Most single player game should in theory survive the death of the developer/publisher as long as Steam is around. Some single player games require a central server for no particularly good reason, or for some functionality that could be disabled without much loss to the game's playability. I think these are the ones that should be called out, and required to either have an end-of-life plan (a flag ready to go that disables online functionality) or a big warning "whelp this game will stop working eventually buy at your own risk".