That depends. Would suggesting they be legal be legal?
Have you encountered situations where ethicists and absolutist free speech advocates have encountered any legal problems for exploring concepts such as this?
You can make slippery slope arguments if you like, but they're somewhat analogous to the very idea you're defending. You're allowed to make meta arguments about curtailing absolute free speech being dangerous, why would someone else not be able to make a meta argument about allowing death threats? They're both equally removed from reality.
I asked, “Are you advocating a system where giving airtime to certain ideas, but not including incitement, would be punishable?”
They answered in the affirmative.
The justification being that entertaining certain ideas itself constitutes harm (because entertaining ideas may be proximate to harm or “increases the likelihood of harm”).
We also got into definitions of “harm”, which, in my view, were expansive. (In the sense that nearly anything could be rationalized as resulting in harm.)