It's also worth noting what, exactly, was the offending speech in that court case, and several related ones (such as https://en.wikipedia.org/wiki/Debs_v._United_States). It was anti-war propaganda, and specifically, calls to resist the draft. The defendants were high-ranking members of the Socialist Party of America.

Principled defense of free speech is often accused of being a slippery slope fallacy, usually by the same people who give "fire in a crowded theater" as a counterpoint. I wonder how many of them are aware that the phrase itself is a historical example of a very real slippery slope.