Conlanging (constructed languages) is, I guess, more mainstream than it used to be. There's an r/conlangs subreddit dedicated to the art. I assume the popularity is due to finding conlangs used in popular films, TV series and video games.
The first conlang forum I stumbled across many years ago was Zompist's ZBB - https://www.verduria.org/ - though sadly the current contents date back only to mid-2018. The original board, with posts back to 2002, is archived at this link: http://www.incatena.org/
The Conlang Mailing List has archives dating back to 1998 - https://listserv.brown.edu/archives/conlang.html
The first website I ever put on the web became a proud member of Scattered Tongues, which was the Conlang webring. Unbelievably, the webring's HQ page is still accessible at http://taliesin.nvg.org/sc/
