Note that this is only the game code that allows people to develop mods, and not engine code that allows people to do ports. The re-release engine code is still proprietary.

They did the same with Quake 1 re-release game code

Just the official version that’s on Steam. I haven’t fiddled with the source ports, so I can’t comment on that.

Recently I was playing Guild Wars 1 and Doom (1996) on it.

Feels great for Doom.

As for Guild Wars 1, it was very much playable. The only frustrations were the login and double clicking in the inventory. Someone had already done a community layout which worked pretty nicely. Note though that I was playing just by myself, so I can’t comment on while in a party with other players.