HN and other "distributed moderation" sites tend to quell any discussion that a fraction of the userbase strongly dislikes. Most people don't vote on most comments, so highly motivated people have more say on a particular topic. In some cases, this is a correction mechanism, such as when people with physics education downvote the proponents of fringe theories; in other cases, it may hide useful information, such as in a debate about morality or a similar less certain topic that people acquire strong opinions about without having such high confidence.
However, I find it hard to justify calling this "censorship" because it is the site working as intended. Controversial topics are avoided because the goal of the site is ultimately recreational. There are many things you can't discuss on HN because the userbase just isn't interested, but nobody would argue that HN censors discussions of, say, the hardcore music scene in the Bay Area.