What is the decisive point for classifying a certain speech as unacceptable?

Question

How did it come to be determined that certain (hate) speech is unacceptable?

What is the decisive point?

Answer 1

The short answer is J.S. Mill's "harm" principle"

In an attempt to use Mill’s own argument against him, critics often cite the frequently misunderstood harm principle. This principle, which states that liberty should be restricted only to the extent that it prevents one person from causing harm to another (e.g. the right to raise my fist stops just short of your face) does not unambiguously apply to speech, and nowhere in On Liberty does Mill articulate which kinds of speech would fall under this category, if any.

The idea that speech can cause or lead to harm is the basis of constraints on freedom of expression in many democratic societies. Hate speech laws, which exist in over thirty countries, are based on the notion that speech could incite violence in potentia. In the United States, legal concepts like fighting words and clear and present danger both recognize this and give it even greater urgancy.

Problem is that the concept/term "harm" has essentially been redefined out of existence:

The concept of harm can be reconstituted to include hurt feelings. We’ve witnessed how institutions will bend––to the point of abandoning first principles––to placate the hurt. And we’ve seen how both these frailties can be harnessed en masse to vindictive ends. What we have then, in effect, is a soft mob, a group that can always appeal to vague feelings of unease to tighten the circle around what is considered acceptable speech.

For instance:

In an internal letter to Vox’s editors––which was then posted on Twitter––Emily VanDerWerff wrote that the inclusion of Matthew Yglesias’ name on the Harper’s letter [https://harpers.org/a-letter-on-justice-and-open-debate/], alongside J. K. Rowling and other “anti-trans” voices, made her feel “less safe” at work––a feeling that Zack Beauchamp describes as “a sort of ‘harm’” of the type that could very well qualify under Mill’s principle.

This tacitly opens up a new frontier, wherein speech itself is considered a kind of violence. In his book The Coddling of the American Mind, Jonathan Haidt talks about how speech is now deemed harmful because it can violate people’s sensibilities, or invalidate their experiences, thus necessitating trigger warnings and safe spaces. In turn, social justice advocates make the argument that they are not attempting to narrow the discourse, but open it to those who have been excluded. But the inclusion of new voices ought to reinforce the principle of tolerance, not weaken it, since the collision with error is valuable precisely because of the plurality of opinion. Instead, this new openness becomes the pretext for certain conversations remaining closed, as new participants make it clear that there are things that will not be discussed.

Both Mill and de Tocqueville agreed that the tyranny of opinion and of feeling (our current emotivist ethos) were potentially even more dangerous than that of a censorious state because they enslave the soul, whereas a brutal sovereign can only command obedience. As De Tocqueville noted about those who utter unpopular opinions:

It isn’t that he has to fear an auto-da-fe, ́but he is exposed to all types of distasteful things and to everyday persecutions … Everything is denied him … those who censure him speak openly, and those who think as he does, without having his courage, keep quiet and distance themselves. He gives in; finally, under the daily effort, he yields and returns to silence, as though he felt remorse for having told the truth.

Sound familiar? Peruse this: https://areomagazine.com/2020/09/03/the-soft-mob-intolerance-and-the-new-definition-of-harm/, from which I derived the above quotes.