If a new Left for Dead installment is on the way, it's news to Valve. The video game publisher and owners of Steam haven't really been interested in making new games with Steam's rise to become the primary digital storefront for PC gaming.
Still rumors of a new Portal or Half-Life game (VR remakes, don't count!) persist, just as the recent online scuttlebut speculated that a new Left for Dead installment has secretly been in development for quite some time.
In an official statement [via IGN], Valve denied those claims. "We’ve seen rumors to this effect for the last couple of months. We did briefly explore some Left 4 Dead next gen 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."
In short, if you're waiting on a new Left for Dead game (that's not some VR remake), don't hold your breath.