Back in February, Valve released the source code for Team Fortress 2 (TF2) into the Source SDK, which allows modders to see how it all works and get their standalone mods onto Steam. But also, it allows them to fix up issues directly in Team Fortress 2.
It’s not “open-source” either, so neither of these communities fit.
Yes, I am aware that the game code can be modified under a custom license now, but that does not extend to the game engine and the license is not OSI compatible, so calling it open-source would be highly misleading.
Many games that can be modded have the source for the game code available 🤷