Valve was actually going to make a 3rd title for the Left 4 Dead series. It never happened because Source 2 couldn't handle it well when it was being tested, and Valve moved on anyways because they were advancing with other software and technology they're working on, like SteamVR and the Steam Deck.
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Early testing of the game proved to be a failure because the game was intended to be an; OPEN WORLD GAME; set in Morocco. But like I said, software limitations with Valve 2 at the time meant it wasn't able to happen, and thus, Left 4 Dead 3 was abandoned because of other projects Valve was working on.
