Why can't we say, "hey, no slurs please" without civilization collapsing into a communist authoritarian nightmare?

Understanding that this proposal is clear, simple, and wrong is kinda the basis of reaching the Enlightenment. But because this is a bit subtle, many people -- too many people -- can't understand it. You have to think structurally, rather than emotionally.

1. Who do you think will be in charge of determining what is a "slur" and what is not? Is saying "It's OK to be white" a slur? How about "Women prefer to work with people, men prefer to work with things"? At the time of the Enlightenment, a slur was basically an insult to the King or Nobility. Which of course applied equally if you were criticizing their judgement/policies, which then grew some more. This was called "Lese Majeste" or violating the majesty of the Sovereign, and evolved into the hate speech laws, for example, in Weimar, that did absolutely nothing to prevent the rise anti-semitism in Europe. Banning hate speech has always failed to protect people against hatred. Now a lot of people think that only those who are oppressed will be protected by bans on slurs, but actually those who decide what is and is not a slur are not just going to be the oppressed. They will be also the ones who already have power, and they will use that ban as another lever to stifle dissent. So, that's one problem right there -- same problem as "Why can't we give this organization vague, broad powers and not have the powers be abused? What's so hard about that?" Well, it's not how life works, and we should know that by now. We should always think about policies with the view that untrustworthy people are in charge, rather than with a simplistic view.

2. In every society, there is an overton window of acceptable views. Society has many, many ways of punishing those who step outside of that window. Everything from social ostracism, to getting fired, to losing customers, etc. You don't really need to worry that society has no way of punishing those who offend them. In fact, it's really a silly thing to worry about. We don't need additional ways of enforcing thought-discipline and speech discipline beyond informal ostracism. Few can stand up to it, and when someone is saying something obviously hateful, those who disagree tend to be strengthened in their resolve against that view.

3. If society already has informal ways of curtailing offensive speech, what is the harm of additionally formalizing more policies or laws that (according to 1.) will curtail speech that elites find offensive? And what is the benefit of not formalizing them? This is what the Enlightenment thinkers understood, but which many have now forgotten. The benefit is when the overton window as espoused by the elites doesn't match with the overton window of society as a whole: in normal situations (both overton windows are the same), you don't need to exert additional pressure on the person engaging in transgressive speech, because they wont find an audience. But what if they do find an audience? That's usually the "emperor has no clothes" scenario, where those in charge of the institution have an overton window that is different from members of the institution, or a sizeable minority of members. In that case, people will listen to the transgressor and say "hey, the emperor is naked!" while the emperor will accuse the transgressor of making slurs. That's the situation in which freedom of speech is so crucial -- when it will resonate and the normal social pressures don't quash the transgressor. This is the example of "It's OK to be white."

So freedom of speech serves as a safety valve from the elites enforcing their bubble. No one cares about actual slurs -- those that don't find an audience -- society will always punish those people. No one cares about freedom to remain within the overton window of the elites. Those aren't considered a slur. All that matters is the ability to mock someone for being naked, when many believe they are naked. But to protect this valuable corrective mechanism, we have to protect all slurs, knowing that what is outside both windows wont find an audience and so we don't care if it's spoken.

But what if a sizeable minority of people really are awful people -- e.g. racists? Wont silencing them teach them to not be racists? Can't the elites force a large chunk of the public into believing something they don't by banning the airing of opposing views? No, they can't.

Part of the reason the Weimar hate speech laws didn't work, although many Nazis were arrested for hate speech, was that seeing people arrested solely for expressing their views gave them sympathy with the public and promoted their cause. Just as the anti-fascist movement gave rise to the SS and strengthened fascism, so the anti-hate speech laws radicalized Germany towards hate. It's funny how well meaning things can backfire, but at some point we need to learn from history. So in that situation in which you have a sizeable minority that is racists, you have to deal with that head on, by letting people speak (and making a fool of themselves), and you have to debate them openly in a war of ideas, rather than by suppressing their speech (and making them appear as unjustly muzzled). If someone who says "It's OK to be white" is really a racist, and they have a podium, they will soon say things a lot less benign, and people will hear that and be disgusted. Others, who also have their own podium, will counter. But if the same person is punished merely for saying "It's OK to be white", then they will be viewed more favorably. So you have to let them speak. They have to be free to air all of their hate, so that it can be debated openly, so people can see it and reject it.

You can't discipline society to be more open minded than they are by policing speech. You can't give moral authority to the racists that they wouldn't otherwise have. No one has ever defeated (or even weakened) racism by banning racist speech.

But if you are only thinking emotively, rather than structurally, you don't see the role that free speech has to play in keeping foolishness in check, whether by letting extremists discredit themselves, or by showing the emperor that they have no clothes. You are thinking simplistically ("X offends me, so why can't I ban X") rather than structurally. And by doing so, you lose an important corrective mechanism and end up with a very sick society as a result. As well as a more racist society.

So the Enlightenment view is to let people speak, and not fret so much about slurs. The slurs will take care of themselves, and will also keep people honest. Society must be one in which there is an open exchange of ideas, especially ideas that many find offensive. It's just a shame that we are losing an appreciation for this view in our war against impoliteness and our forgetting of our history.