Even though I don't have a reddit account, I do check into anarchist or leftist subreddits from time to time. Here is a list of them for those who are probably interested:
General leftist subreddits
/r/Socialism
/r/Socialism_101
/r/CapitalismVSocialism
/r/ChapoTrapHouse
/r/communists < this one is an anti-tank forum. It's dominated by leftcommers I think, but there are anarchists. There's not a lot of activity at the moment though.
/r/LateStageCapitalism
/r/IWW < Industrial Workers of the World
/r/Communalists < this one is a specific tendency obviously but it doesn't into either anarchist or Marxist
Anarchist subreddits
/r/Anarchism
/r/COMPLETEANARCHY < memes
/r/Anarchy101
/r/DebateAnarchism
/r/LibertarianLeft
/r/LibertarianSocialism
/r/anarchocommunism
/r/anarchafeminism
/r/socialanarchism
/r/anarcho_syndicalism
/r/anarchosyndicalism
Anti-Statist Marxist subreddits
/r/leftcommunism
/r/Situationism
/r/Autonomia
/r/Luxemburgism
/r/communization
/r/workerscouncil < council communist
I should also mention /r/Ultraleft and /r/LeftistHotTakes, both of which were set to private not too long ago.
Not very active
Not very active but
/r/impossibilism
/r/worldsocialist
Thanks. Might also be worth
Thanks. Might also be worth making a mastodon instance list. I haven't come across any decent communist instances yet. The one anarchist instance I know of is not the best discussion-wise and has market-advocating "members" (if that's what you call them; still haven't entirely figured mastodon out yet).