I feel like this is similar to the death penalty debate though, where in response to the concern about executing innocent people, you hear "we'll only use it when it's a clear cut case" but either it ends up being used more than that, or it'd be used so rarely that it's not worth carving out a special case for it. How many things are on the clear cut level of Holocaust denial? And who decides what's clear cut enough to be excluded from free speech?

>clear cut level of Holocaust denial

'Denial' isn't clear cut at all. You could deny it by saying it didn't happen at all, or that it happened in a different location, or by different people, or that the number of dead doesn't match the stated total(s). Is it denial to disbelieve a specific person was killed, or was killed in a specific manner? Is it denial to disbelieve some of the particulars of someones story? Free speech means exactly that; you're free to speak or you aren't.