Valve, is a platform. They have little reason to remove games themselves. But they do work with, and comply with, video game corpos doing this horse crap (not to be confused with horse armor which is also still crap). So they’re at least complicit. See OP.
The core issue is that “buying” on any of the platforms (except GOG) is not buying but leasing for a one time payment. While Gaben is pretty altruistic it’s only a matter of time until someone less so is in charge and decides X isn’t wanted anymore and then poof because the EULA (also subject to change at any moment) allows it.
This is the key thing now. Apart from places like GOG you don’t really ‘own’ games. You lease a license, that can be revoked. Or in the case of the ‘always online or have to connect to whatever server’ you get that game until they kill the connect server, even on single player games. Steam is at least decent (for now) at keeping those still yours, but as mentioned all it takes is Gaben to no longer be in charge and it will go away too.
Some games still work and just give you that failed to connect, others are just dead and can’t be played ever again. Some certainly still work after severs shut down but others are dead.
The days of decade plus old games still working is starting to fade away and it sucks.
Minor point but its all technically down on the developers. You aren’t forced to use Steam DRM and several games don’t use it at all. For instance Witcher 3 on steam is DRM free and it functions basically as it does on GOG. Where you can just move the game to wherever and launch it.