No, it's not an announcement or even an admission that anything is happening over at Valve regarding Left 4 Ded 3, but writer Chet Faliszek has suggested that it's certainly a possibility, noting that it's one of his favourite series.
"A 3? You know, some time down the road," he said when asked about its possibility, chatting to Eurogamer at EGX. "Like anything, I would say that to all of our games. So that's not a promise of when or where."
Asked if it's a series he'd personally like to return to, Faliszek's answer was positive. "I really love working on that series. It's one of my favourite series. It's just so much fun with the characters and the world. I love zombies and I love horror. So, yeah."