ACX often has good discussion in the comments (but the lack of voting makes it quite hard to find them)
AskHistorian subreddit
There definitely exist good Twitter conversations but no good way of finding them. I do think following Gwern, Eliezer, Ajeya, Emmett and a few others on Twitter does tend to surface high-quality discussion
Hacker News sometimes has good discussion, especially when the authors of a linked article show up
Some other places off the top of my head:
ACX often has good discussion in the comments (but the lack of voting makes it quite hard to find them)
AskHistorian subreddit
There definitely exist good Twitter conversations but no good way of finding them. I do think following Gwern, Eliezer, Ajeya, Emmett and a few others on Twitter does tend to surface high-quality discussion
Hacker News sometimes has good discussion, especially when the authors of a linked article show up