Will Valve ever release Left 4 Dead 3? Probably not. However, at one point, it was not only planning on making a third entry in the popular zombie shooter series, but a third installment was in active development. In fact, Left 4 Dead 3 wasn't just in development, but it sounds like it was an incredibly ambitious project. Further, it was poised to evolve the formula into an open-world game.
According to Geoff Keighley's new documentary on Half-Life: Alyx's development, The Final Hours of Half-Life: Alyx, Valve was working on Left 4 Dead 3 for a while, and it was an ambitious project. While the game was designed to be another co-op shooter, it was also going to be an open-world game, which would be a big change for the linear level-based series. But it wasn't just going to be a basic open-world game; the plan was for it to set a new bar within the genre.
The documentary mentions the game was aiming to have hundreds of zombies on the screen at the time. In a post Days Gone and World War Z world, this may not seem that impressive, but at the time, it would have been.
Lastly, Keighley reveals the game was going to be set in Morocco, which would be a unique setting for a video game. Not many video games are set in Northern Africa, let alone big AAA games.
So, what went wrong? Well, the documentary mentions that the game's development ran into issues due to problems with the engine the game was being built upon, Valve Source 2 engine, an engine that Valve eventually dumped itself, bringing with it a few game cancellations in the process.
Of course, everything here should be taken with a grain of salt, but there's no reason not to trust Keighley's information. Not only is he a very reliable source, but Valve presumably signed off on everything in the documentary before release.
Again, will we ever see Left 4 Dead 3? Who knows, but it's safe to assume we will never see these version of the third installment.