A figure of speech meaning what? Most people, AFAICT, that use it use it as an widely-perceived authoritative example of something specific that is accepted to be outside of the protection of free speech, a use that derives from and its use in the Schenk v. U.S. decision (it is sometimes explicitly described as something the Supreme Court has declared as outside of the protection of the 1st Amendment, which clearly derives from that origin.)

Of course, reliance on it for that purpose has problems because (1) it was dicta, not part of the ruling, in Schenk, and (2) Schenk is a notoriously bad decision impinging on core political speech in its specific application, and whose general rule is also no longer valid.

I have no idea what it would communicate as “a figure of speech”, and if it is actually used by some people as a figure of speech meaning something other than what it literally says being an example of unprotected speech (including, though I can see how this use would have some logic, as a figure of speech meaning “a persistently popular, despite being notoriously wrong, understanding of a legal rule”) it is one that impedes rather than promotes communication.