[Symbiote]

If a comment is downvoted sufficiently, it becomes "dead" and is no longer shown by default.

You can reveal it by going to your user settings and changing "showdead" to "yes".

[ars]

No, that's not from downvotes, that's from "flagging".

Personally I only flag spam and hate speech, but some people get really upset by certain posts and will flag them rather than (or in addition to) downvoting.

With enough flags the post becomes "dead" and then you can't reply to it, and it's hidden by default. It's possible to un-flag it with the "vouch" option.

I think flagged posts are often reviewed by dang (a moderator).