I'm not just talking about the "flame-war detection", although that system alone is extremely crude an ineffective (in my opinion).

There are lots of other moderation features -- e.g. shadow bans and automatic vote penalties -- that are completely opaque but turn HN into a strong echo chamber.

I also disagree with your point about Reddit. At least on Reddit you can easily track what was removed (there are entire subreddits dedicated to tracking what's been removed by moderators) and many subreddits provide explanations of why content was removed.