There's a difference between censoring and curating.

Popular subreddits don't seem to to curate for meaningful discussion. So jokes, memes, and other low-effort comments often make discussion on reddit close to worthless.

The exceptions are always either small niche subreddits where a common deep interest focuses the discussion, or they're curated subreddits e.g. the neutral politics subreddit.

And as for censoring, moderators are often accused of selectively enforcing rules to favour their agendas etc. I must admit I'm not sure how much that actually happens though.

Its more that it allows communities to interact negatively with each other too much. Generally speaking nobody in the "toxic" communities is worried about the speech there or they would unsubscribe. The problem is when they brigade on other subreddits, or people not OK with what is said in one community decide to impose their morals there. The net result is large communities at war with each other.