Well it's not known to me. What makes you say that? We don't delete comments or posts like this, but if you know of specific cases we can certainly take a look.
You...do appreciate the difficulty in finding these instances if they weren't bookmarked at the time, right? Also, just how silly it is to have the alleged censors say "Oh, well, that's news to me--can you find examples of the things we memoryholed?"
It might help if you folks kept a public moderation log.
I don't think it's unfair to ask someone to back up a claim, especially when the dataset is publicly available through the API or (more conveniently) in BigQuery:
SELECT count(*) FROM [bigquery-public-data:hacker_news.comments]
WHERE parent IS NULL;
Story and comment IDs are also simply incrementing integers. We're not making it easy on ourselves to hide this sort of thing!
It might help if you folks kept a public moderation log.