Just saw that my question was closed for, apparently, being opinion-based.
Therefore, I wonder what makes a question an opinion and why would my fit that category.
I do see this question as objective.
Yes it is because what you may consider acceptable free speech someone else may consider unacceptable. Asking people on how to classify what is and isn't acceptable is going to get many different opinions on what should be allowed or not. A good example of this would be the debate with speakers on college campuses and how some of them are not allowed to speak because people don't want to hear what they have to say.
The title of the question is opinion-based.
What is the decisive point for classifying a certain speech as unacceptable?
The fact that there are very different laws and regulation around the world about what is and is not acceptable speech and that even in those countries the court rulings are all over the place should tell you that this is a question which has no right or wrong answers. Different people have very different opinions on the matter.
But the question asked in the body of the question is actually a different one:
How did it come to be determined that certain (hate) speech is unacceptable?
This is actually a question which could be answered, but as written like that it is too broad to answer. As I said, different countries have very different views on what speech is unacceptable and did arrive at that consensus in very different ways. So you might have to find a more specific example. If you asked us "what is the official rationale for law X in country Y", then that's something we might be able to answer.