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That said, the Discord section of the diaspora map has gotten pretty out-of-date; a lot of the chats listed have either died, gone dormant or been subsumed by others (or, indeed, have increased in stature greatly through the process of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 a compelling narrative). 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 rats' cords "a compelling narrative" and "LessWrong", the former of 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.
In its ideal, most reasonable form, and even in the diluted form espoused by rat[ional]ists, I don't personally have any issue with the concept of growth mindset in and of itself. It seems no different than any other form of naïve optimism, power-of-intention handwaving, or placebomancy -- but precisely because of this, I feel it falls short of its mythical status as received wisdom.
I've noticed that rats have a tendency of venerating concepts like this in a scope that far outstrips what the original (evidence- or logic-based) formulation was used to denote. To me, there's no big problem with that. All groups of people share some sort of mythos, or things that "everyone knows". Demanding that all of these sayings and mottos and dank memes be based on dispassionate statistical analysis is not only unreasonable, but an obstacle to making progress.
However, I think that we had better become mindful of the fact that our memes are not substantially better, and we are, most of the time, not substantially wiser than members of any other subculture when we try to apply them to reality -- or we run the risk of digging ourselves into antics far more absurd than even the most foolish "normie" could dream of!