As you requested, I leave a comment concerning the downvote.

I think it's appropriate to flag posts/comments that deny well-established scientific or historical facts, examples include Holocaust denial (see also here), Climate science denial (see also here), Flat eath etc.

Also, I prefer to avoid discussions with deniers about the topics of their denialism, because it gives the impression that there is a debate where in fact there is no debate. And it's not worth my time to debunk the denier's arguments that have already been debunked hundreds of times before. While some deniers are just victims of disinformation, lack education or intelligence, or suffer from some mental condition, others do it to further a political agenda, for the purpose of trolling or for profit.