Hi! There's been a couple of good answers so far. As far as I'm aware, the Russian LW slack is very much alive and active, and as far as I know namespace's diaspora map is the most comprehensive list assembled as of now. The LessWrong IRC and Slack, as far as I know, remain active and populated by many intelligent and worthwhile conversations.
That said, the Discord section of the diaspora map has gotten pretty out-of-date; a lot of the chats listed have either died, gone dormant or been subsumed by others (or, indeed, have increased in stature greatly through the process of subsuming). I would update it, but I don't appear to be able to edit the page.
While a comprehensive conspectus of the rise and fall and death and rebirth of "rats' cord" is far beyond the scope of what I can type out in a single post, and while I am indeed currently writing up a post that goes into excruciating detail of statistics kept over the last two years (including graphs!) I can at least give a brief overview right now.
There have been, by various estimates, anywhere between ten and a hundred "rat discords" created over the course of the last two years. As of 2019 March 31, there are only three with a monthly average of more than two thousand lines posted per day (to wit: slate star codex, r/rational and mine). Interestingly, these are also the only three with a monthly average of more than one thousand lines per day.
This is by no means a comprehensive assessment of the social landscape; obviously, many factors besides line count determine the quality and atmosphere of a space, and it's indeed the case that many chats -- such as the EA Corner, MIRIxDiscord, and LessWrong -- maintain a much smaller presence despite being in many ways more centrally 'rationalist' than their more-active cousins.
My apologies for this post being concise to the point of uselessness -- there is a great wealth of other relevant things I've picked up over the last two years of administrating spaces in this area and plan to articulate soon -- but I've got plans this evening and am only able to write up a quick summary right now!
PS: In the interest of full disclosure, I am the founder and proprietor of my 'scord and the 'scord of "LessWrong", which I've linked to in this post.
PPS. I am not sure what the policy is on posting public invite links to certain chats, and the LessWrong discord itself is currently in the process of a renovation, so I have omitted some from this post, but would be happy to update it once I have gained permission from the relevant people.