Not forks. The whole code for Proton is in https://github.com/ValveSoftware/Proton and its submodules. There are many game-specific patches, i.e., patches that were more or less explicitly included to make one specific game (or a few games) work better.

There are also some instances of game-specific behaviour: Proton detects at runtime which game is running and decides whether to enable or not some code. This is done for the most filthy hacks that are required to run something, but risk to spoil some other program. Search the code for instances of getenv("SteamAppId") or getenv("SteamGameId").