No real forum out there that I've come across. There's a PL slack that's not very active. Most of the discussions I come across is just from following a lot of PL authors on http://scholar.google.com, and from Twitter. Wish there was a proper forum!
there's a Discord server populated by hobbyists and a few academics. there are a lot of very knowledgeable members and they welcome all manner of discussion!
the server operates in light cooperation with the /r/ProgrammingLanguages subreddit, which is maybe more of a traditional "forum" experience. again, mostly hobbyists from industry, but also some academic posts/discussions now and then!
I think the cool thing about the PL discord at least is that it gives folks the opportunity to mix with circles they wouldn't normally. Eg. researchers, industry people, and hobbyists. That's great for helping to transfer ideas and break down barriers.
But yeah, it's not so great for long term stuff, like I still find myself coming across LtU threads in my searches years later… you don't get that with discord. 🙁