Actually, that's not true. I entered a mildly critical comment on a YouTube video the other day, very spot on if I do say so. It immediately got a bunch of upvotes, then that stopped completely. My comment was on top still, but that's just how YouTube displays your own comments.

But when checking the video without logging in to YouTube, my comment was gone.

So it looks like comments can be quietly deleted without the commenter even knowing it.

THAT is censorship.