This is something I have to disagree with. I don't think holocaust denial
when the opinion is expressed without vitriol or racism can be equated to the same types of slanderous stereotypes that harass minorities as your other examples.
I get what you're saying: some opinions are 'bad', and ought not to be supported on GAF. However, I feel there's a huge distinction between attacks on a group of people expressed as objective truths ("gay men are all sexually immoral", "Jewish people are greedy" etc); and a crazy belief in a claim that most of us know to be historically inaccurate. Just my opinion, but I don't think Christian Fundamentalists should be banned for saying "according to my beliefs/in my opinion/the Bible says menstruating women should be shunned/homosexuality is immoral/he that is wounded in the stones, or hath his privy member cut off, shall not enter into the congregation of the Lord." Expressing an opinion as one's perspective in this way is totally different to trying to pass it off as a statement of fact.
Holocaust denial is tricky, because it's bound up in anti-semitism. However, it's entirely possible for someone to believe they are in possession of knowledge that calls the accepted history into question without also harbouring any ill-will towards Jewish people.
Amongst the Ukrainian diaspora, there is a tussle going on regarding
Holodomor, the soviet-induced famine that killed millions of people in the early '30s. Most Ukrainians want the event to be officially recognized as a genocide. They feel it was a deliberate attack on the Ukrainian people by Stalin, with intent to wipe them out. Some historians support this claim. Many other historians, however, argue that the famine was not a genocide, because Stalin didn't care one way or the other about the Ukrainian people: their deaths were a byproduct of his policy, not his goal. More on this subject
here.
If we lived in a world that generally accepted the idea that Holodomor was a genocide and that every historian who claimed otherwise was an anti-Ukrainian racist who shouldn't be given a voice, we would be wilfully ignoring a great deal of evidence that paints a clearer picture of what actually happened.
Now, I'm not saying for a second that holocaust denial has the same intellectual value as Holodomor-as-genocide denial. I'm just saying I think all opinions should be held up to scrutiny and dismissed based on evidence rather than disgust. I don't think holocaust denial is distinct from the belief that 9/11 was an inside job, or that Queen Elizabeth is a lizard-woman. Those opinions are provably, factually wrong, but they're not verboten on GAF.
While I agree that anyone who actively encourages misogyny, homophobia, anti-semitism etc. ought to be slapped with a ban, I also think it's important to draw a clear line in the sand between explicitly propagating false stereotypes/information with intent to harass, and the various weird opinions that the majority of people view as equally unacceptable.
All that being said, if the example you were thinking of said something like "guys, the holocaust was exaggerated - we all know that, that's something science has taught us", then that's propagation of false information with damaging consequences, not an expression of a personal opinion. That's a ban I can get behind.