Valve adds "all the Team Fortress 2 client and server game code" to its Source mod tools, letting modders "build completely new games based on TF2" and publish them on Steam

https://www.gamesradar.com/games/fps/valve-adds-all-the-team-fortress-2-client-and-server-game-code-to-its-source-mod-tools-letting-modders-build-completely-new-games-based-on-tf2-and-publish-them-on-steam/