Still, this is Valve – a company that made its name with games, not Steam, so they have likely not forgotten where they started, and could be preparing something as a surprise to their fans. They would probably not make the official announcement until the project is close to completion though, knowing their style.
“Left 4 Dead 3” could definitely be a great game to promote Source 2 with as well, and before the rumors about its cancellation, there were claims that Valve are preparing it as a free-to-play game with an experimental business model.
If Valve have no intention to work on the game though, they should probably give permission to Turtle Rock to take over its development at some point in the future, as it would be a shame to see the franchise fading into obscurity simply because nobody wants to develop it anymore. Zombie games have been overdone in the last few years, but it still has potential.