IANAL, but generally, the speech that is limited in the US is to things like yelling "Fire!" in a crowded theater - i.e. things that might cause general panic.

IANAL, but generally, the speech that is limited in the US is to things like yelling "Fire!" in a crowded theater - i.e. things that might cause general panic. Speech such as the Holocaust didn't exist doesn't represent immediate harm, and as despicable as it may be, is protected.