The GTA case is because js technically an update from Rockstar, not specifically Steam's fault here. There are always to downgrade a game, but true, it is not as easy as GoG.
No it's not steam's fault but they do allow it to happen.
Would they not allow it? Rockstar is the owner of the game, they can update any removal of content if they want to. Modern games frequently change gameplay, story, and soundtrack between versions