How is that not an exception, though? The test 'whether the speech will provoke immediate face-to-face violence' is still a restriction on free speech, albeit an extremely circumscribed one. Which is exactly what we were talking about, that such circumscribed exceptions are possible in the first place without sliding all the way to 'criticizing the government is treason, citizen'.

But, ok, if you want another U.S.-based exception: some information regarding nuclear weapons is considered to be 'born classified' in the sense that even if you develop it on your own without clearance or access to classified materials, you still are not allowed to divulge it. That is another government-enforced restriction to free speech, although admittedly a sensible one on the justification of the survival of the species.

Don't get me wrong, I think in any specific discussion about speech restrictions the bias should be huge in favor of free speech, given the obvious dangers of any too broad restriction and the incentives of people in power to put in place such restrictions. But we definitely don't currently enjoy, let's say "absolute information-theoretical freedom of speech" (e.g. including things like direct actionable threats, weapon blueprints, video-recordings of certain third-party crimes, etc). We already put up with some limits without falling into a slippery slope. 'No explicit calls for genocide' might not, on its own, be the restriction that opens the floodgates to pervasive censorship.