A new L4D game is not in the works, Valve says, despite rumors of Left 4 Dead 3 for VR.

While Valve is returning to the Half-Life franchise after more than a decade with its VR-exclusive Half-Life: Alyx this year, the company says its absolutely not developing another heavily rumored game: Left 4 Dead 3.
In a statement to IGN, Valve said that it briefly explore[d] some Left 4 Dead next gen opportunities a few years ago, but that its not working on a new Left 4 Dead now and hasnt done so for years.
A new Left 4 Dead game has been rumored for close to a decade, mostly in the form of leaks and hints excavated from files and source code in Valves Source game engine. Some of that work in unearthing references to Left 4 Dead 3 has been done by Tyler McVicker of the Valve News Network YouTube channel. McVicker has published videos on asset leaks and other information about scrapped Left 4 Dead projects, including one that he reported was canceled in 2017.
But it was McVickers recent tweet about a Left 4 Dead VR game that stoked speculation about the games existence and imminence. In December, McVicker tweeted that Left 4 Dead VR is real, and with the recent confirmation of Half-Life: Alyx, that tweet was widely interpreted to mean that L4D VR was in active development. In a new video posted Friday, McVicker explained that his reference to Left 4 Dead VR had to do with a prototype that was in development in 2017 that I had found out about and have been working on a project for.
Thats all I said on the matter, McVicker continued, saying that his comments were misconstrued by gaming websites.
They were also apparently misconstrued as fact by the China president for HTC VR, Alvin Wang Graylin. On Friday, Graylin tweeted slides from a presentation he was giving on VR trends in 2020. One of those slides included the line Valve HL Alyx / LFD3 will drive consumer and AAA studio interest, leading some to believe that Graylin had inside information on a new Left 4 Dead, given Valve and HTCs previous relationship.
But Valve has uncharacteristically squashed those rumors for now. In the meantime, Left 4 Dead fans will have to remain content with the first two games, or one of their upcoming spiritual successors, such as GTFO or Back 4 Blood, the latter of which is being made by original Left 4 Dead developer Turtle Rock.