Rumours had started circulating recently when the president of HTC China, Alvin Wang Graylin, shared some slides on the future of VR on Twitter, with one that stated "Valve HL Alyx/LFD3 will drive consumer and AAA studio interest.".
Suffice to say, this seemed to start a fire among fans longing for a return to Left 4 Dead, seeing this as a seemingly accidental leak that Left 4 Dead 3 could be in the works, presumably for Valve's VR platform much like Half-Life: Alyx.
A few slides from my talk tonight... pic.twitter.com/bHegijjZW1— Alvin Wang Graylin (汪丛青) (@AGraylin) January 17, 2020
It seems though that the gun was potentially jumped a little, as it seems rather than having any insider knowlege, Graylin was simply quoting rumours shared on the Valve News Network, a popular fan website for Valve games.
Perhaps something was list in translation along the way, as Valve have flat out denied such a game exists or is planned;
We’ve seen rumors to this effect for the last couple of months. We did briefly explore some Left 4 Dead next opportunities a few years ago. But we are absolutely not working on anything L4D related now, and haven’t for years. It’s clear some people are having fun creating misinformation to spin up the community and other outlets. Unfortunately, for now a new L4D game is not something we’re working on.
So while Valve haven't ruled out something the future, there's certainly no immediate plans and Left 4 Dead will remain dead and buried for now.