It makes the opinion hidden from the vast majority of visitors, except those who go out of their way to both have an account and showdead.

It is in every appreciable way censorship via unaccountable mob. It's censorship in a way that Reddit's downvote isn't, because Reddit allows anonymous users to read downvoted posts - or at least did the last time I checked.

Does your definition of censorship require everything to be viewable by anonymous users?

Does your definition of disagreement require that it be made harder for others to hear voices you disagree with?

That was an illuminating question. Thank you. I see my parent's point now because no, I don't think that disagreement requires what amounts to censure if it makes it harder for something to be seen.

Pretty much.

At the point where tensions rise beyond polite disagreement, HN ceases to be a functional social space and turns into a game of "who can make the other's opinions disappear first."

Doing so is technically against the rules, but either the moderators don't care, aren't doing it on a large enough scale to be an effective deterrent, or are knowingly complicit.