Obvious example: “I’ll kill you if you don’t shut up.”

Oh, ok. But, I thought they were referring to the term freedom of speech as in the first amendment (not includes obscenity, child pornography, fighting words, and the advocacy of imminent lawless action) because they've given an example of cyberbullying

"I'll doxx you if you don't shut up"? Or, more generally, a constant barrage of things like "kill yourself" or "you're a [insert slur here]", which doesn't rise to a First Amendment violation but definitely has the effect of silencing people?

Imagine dozens of people calling every comment you make wrong, unprofessional or dumb.

There aren't many people for whom this would have no effect on the nature and quantity of their comments.