The idea that ‘free speech’ or ‘free expression’ is a dogwhistle for the far right (or fascism or whatever word you want to use to shut down your audience's higher reasoning functions) is an idea that is as worthy of consideration and has as much to offer us as the ideas that ‘privacy’ and ‘due process’ are code for hating America and that the people who invoke them are complicit in terrorism.
It's as cogent and valuable as the idea that free speech is a dogwhistle for supporting Joseph Stalin and that the people who appeal to it are complicit in sending people to the gulags and corrupting our precious bodily fluids. (People often reject the comparison between now and the anti-communist hysteria of the 50s on the grounds that nationalist authoritarianism and communism are completely different. As someone who wants to deconstruct and abolish the concept of property, I agree. They are completely different. However, during the height of anti-communist hysteria, most communists in the world supported the Soviet Union under Stalin, which was not noted for happiness, sunshine, and puppydogs. Those defending their freedom of speech were still in the right and those attacking it were still in the wrong.)
It's as coherent and worthy of attention as the idea that opposing the draft is a dogwhistle for supporting America's enemies and anyone who does so is complicit in whatever the military was sent out to stop.
It's the childish instinct of “Ewwww! They got cooties on it!”
The same “I used the word ‘safety’! that means you have to stop inquiring into whether anyone is made safer to a degree that justifies the abrogation of competing interests or investigating any potential negative consequences!” from the time after 11 September 2001 now recapitulated as farce.
The dustbin of history is too good for it. It needs to be washed under a tide of contempt into the Sewers of Oblivion.
Also: “But this isn't a state, so it can't violate anyone's fundamental freedoms!” is an excellent argument to make if you and the person you're trying to convince are both radical capitalists. If you are not a radical capitalist, you may wish to find an argument that is not completely inconsistent with your entire political worldview.
“It's their private property, they can do with it whatever they want.” is also an excellent argument if you happen to be a radical capitalist. It's less excellent if you use it to justify Youtube's removing gun demonstrations one day, but then you post an article castigating Youtube for marking all gay content as 'Adult' the next day, and one criticizing social networks for silencing marginalized voices the day after. This doesn't necessarily mean that Youtube shouldn't remove gun demonstration videos (though it's hard to think of a reason why they should), nor does it mean that it's okay for Youtube to mark gay content as adult (it isn't), or that it's okay to silence marginalized people (definitely isn't). It means that there is no consistent set of beliefs you can while making this argument and objecting to other acts and you have a shameful habit of reaching for the nearest no-thought rejoinder that might silence people you dislike.
“Freeze peach!” is a truly excellent response if you're six. It may make people who already agree with you about everything cheer along with you, but people who have points make them rather than misspelling things they don't like.
“The first amendment only restricts what the government can do!” is an excellent argument if someone appealed specifically to the first amendment and not to the general importance of free expression— or if you believe right and wrong are determined only and exclusively by the text of United States law.
“Nobody has a right to a platform!” is an excellent argument if you're the kind of person who believes that everybody has the right to free trade, but nobody has the right to the means of life, shelter, or production. (Who knew that so many self-proclaimed anrcho-communists were actually radical capitalists!) It also requires that you've never appealed to the positive right of expression (i.e. the right to a platform) gained when hate speech and harassment are removed from an environment or complained of people being silenced.
What should you do instead?
First you should give reasons to believe that your proposal will actually have a concrete effect. The argument that strong anti-harassment measures protect people's positive right to expression? That's actually a good argument! There's a lot of reason to think that's the case! Can the same claim be made against hate speech generally? If it's about giving people the tools to avoid being exposed to it, yes. Otherwise, it's difficult to say. European-style hate speech laws seem to be a waste of time. There's little evidence they make Europe any less prone to nationalism or hate or anti-immigration sentiment or antisemitism or any of the other stuff you'd want them to restrict. There is increasing reason to think that online echo chambers may contribute to racist and anti-immigrant terrorist incidents, but even then not everything you could do about it is that likely to help. Germany's attempt at curbing online hate speech looks to have done little.
Second, you have to contend with whether your proposal has undesirable effects. Every proposal for dealing with online hate speech that has been enacted or come close to it is terrible. Why? Because it creates a censorship regime overseen by corporations, with little transparency or oversight, insufficient remedy in case of false accusation, and perverse incentive structures which far-right groups are happy to use to silence their critics. I said that's what would happen if some of the proposals the EU was considering were enacted, and that's what happened with Germany's law. Videos attacking the far-right are regularly taken down by the same platforms on which people continue spouting racist and homophobic invective.
I see someone saying “Something must be done! This is something!” What you're actually saying is that you'd like to spend political capital and give people the feeling that the problem is solved on a supposed remedy that doesn't remedy anything. There is a reason why many marginalized scholars, particularly in Europe, view hate speech laws as a form of tokenism. They make the majority feel as if the issues of bigotry have been addressed and offer a sense of virtuous accomplishment for having taken stern action, while nothing has been accomplished. Please, go join your soulmates who pushed through the PATRIOT act. And you, saying “People are in danger right now, and you're worried about future concerns? For shame!”. The amount of anti-fascist and anti-racist material that's been taken down under poorly incentivized anti-hate policies is well documented. You'll find the USA PATRIOT Act supporters good company as well.
Third, the slippery slope argument is only sometimes a fallacy. When someone is arguing for some sort of moral or ontological continuum: that if you put one thing in a category some other things must follow it there? Fallacy and garbage. When someone argues that a course of action is likely to lead to a negative outcome? The strength of the argument depends entirely on the justified degree of belief in the outcome.
There is a system in the UK called CleanFeed. It was a secret, unaccountable list of sites that ISPs would block, notionally to keep people from downloading child pornography. You may like the idea in itself. You may not. However, the existence and use of CleanFeed was cited by a judge to justify their decision that ISPs were required to censor sites that the publishing industry claimed were havens of media piracy.
In this case, people were perfectly justified in arguing (as some did) that CleanFeed would enable future censorship regimes. This kind of thing is the reason why some people who identify as anarchists and communists, both groups that were treated quite poorly in the past because of their politics, may be less enthusiastic than you are about jettisoning the norm against discriminating against people because of their politics. Whether you think they're justified should depend on how confident you are that the ‘centrists’ and ‘moderates’ you pour scorn on and whose intelligence you deride are insightful enough to see that different ‘extreme’ ideologies aren't equivalent in the same way that you do. This doesn't mean it's necessarily wrong, but it does mean that more justification is needed than jumping up and down and calling anyone who disagrees with one a fascist sympathizer. Focusing specifically on, for example, advocating violent repression or exclusion of some group is way more defensible than the attempts to more broadly claim that this or that is “code for” or “motivated by” something else.