https://wccftech.com/left-4-dead-3-leak-htc-vr-exclusive/

Update: Well, that was certainly fast! Valve has issued a denial, saying they haven't been working on anything Left-4-Dead-related for years...

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.