Valve: Left 4 Dead 3 Not in Development

IGN has an official statement from Valve saying Left 4 Dead 3 is "absolutely" not currently in development, following the recent reveal of concept art for a new installment in the co-op zombie shooter series. Here's the full statement:
“We’ve seen rumors to this effect for the last couple of months,” Valve said in a statement. “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.”