Who is being stopped from saying anything which they want to say?
Good question.
Let's sharpen that stiletto a bit, shall we? What IDEA couldn't be expressed, to the effect it cannot be defeated in the realm of proper debate?
Moreover, once this idea had been expressed and was actually defeated (assuming such thing actually exists), what would be the consequences?
Crickets.
To the surprise of exactly no one.
Of course, we all know what kinds of "ideas" are being met with the most opprobrium: It's everything surrounding the issue of "Their" "inferiority", asserted, or implied. And that's why we see predominantly reactionary White males screeching their pinheads off about "authoritarian" notions of PC and the egregious imposition Those people be treated with respect. Also, outside of academia, there is no such thing as a "defeated idea" - the public discourse is populated by zombie ideas galore, defeated decades ago, but still shambling along to this very day, ranging from "tax cuts pay for themselves" to "women can't control themselves" and way beyond.
The letter writers make it abundantly clear, they are out there to defend their professional liberty to write whatever they please, dumb mistakes included, without repercussions. So far, so good. Everybody is free to be as self-serving as they want. They aren't the ones carrying the additional burden if Rightardia runs with their notion of freedom of speech and re-normalizes the N-word (and similar zombie-"ideas"). That would be Those people, returned to the 1950s, getting their daily dose of denigration and humiliation. Not a single word about Them to be found in that fabulous letter.
Now, let's not forget how suppression of ideas actually works, and how danger ensues subsequently: Everyone who viewed, rightly, Trump as a crook and an incompetent clown is gone from this administration, and the Yes-Men have taken over. Everyone who dares to expose Trump's disinformation on Covit-19 is the target of Rightardia, and urged to be fired, such as Dr. Fauci, who has been sidelined and muffled already. The CDC are being pressured to rework their school opening guidelines because the Orange dunce deems them too restrictive and a hindrance to his reelection plans. People are actually dying and even more are going to die because of that. There is real danger in suppressing ideas - yet none in giving long-defeated zombie ideas and their promulgators and hateful propagandists short shrift. Also, there is no danger involved in corporations canceling ad buys because they don't want their product be associated with White supremacists and other Nazis. But it's exactly the right kind of people screeching about "freedom of speech" on this occasion. The 1950s have called. I hear they want their village idiots back.