https://www.reddit.com/r/communism/wiki/debunk It seems to "debunk" a lot of atrocities and such.
Heads up: you're more likely to get a high quality response if you select one or two general claims in that list and make a post explicitly naming them in your title.
For instance here's a thread about Stalin's role in the Holodomir (Ukraine famine) with /u/TenMinuteHistory 's response.
https://www.reddit.com/r/AskHistorians/comments/536qpy/stalin_seems_to_get_lots_of_blame_for_the/
edit: see also /u/jschooltiger 's aggregation here on Holodomir
this sort of general "debunk _____ apologetics" post both isn't loved by flaired users (based on AH polls) and will not trigger any keyword triggers set up by users. If you're feeling really stuck you can also just message the mods about how to frame your question in order to maximize chances of a good response.