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".