As objectively abhorrent as anti-semitism is, I unfortunately think the issue is not so cut-and-dry for a variety of reasons.
First, just to respond to those 2 points:
1. There's a spectrum of anti-semites. On many parts of the Internet, you will see many anti-semites who won't even hide under the veil of "just joking". They will tell you to your face that they sincerely believe that either the Holocaust was a hoax, or that it was a good thing. (Or it was a hoax but should happen for real.) Many of these people don't come to these beliefs purely through childhood indoctrination but rather exposure to communities and information sources which, to them and in the context of their existing worldview, seem rational and accurate and polarize them further. When you debate an anti-semite on the Internet, you'll often find that a good percentage don't argue in bad faith and actually do cite what they consider to be credible evidence of their claims. This is in the form of links to websites that look superficially scholarly and professional, images and infographics that claim to be backed by studies, and long documentary-style videos. No, the evidence is not good at all (and any accurate information they do manage to present may support something or other but doesn't support their anti-semitic claims), but many genuinely and truly think the evidence they're linking you is damning.
These aren't all random idiots purely looking for an excuse to hate on Jews. Many are brainwashed from what they read and are exposed to, just like any extremist ideology or cult. They're conspiracy theorists.
You can't fight that sort of thing with censorship: you're proving their point when you do that. "They want to hide this information because they know we're right. They're trying to hide The Truth from the goyim masses. We have facts and data on our side, and all they can do is censor and ban us." As futile as it almost always is to actually have a logical discussion or argument with such an individual, rhetoric must be fought with rhetoric, plain and simple. You can't fight rhetoric with "your argument is too ridiculous or evil to be dignified with either a response or even an allowance of existence".
2.
>However, it is not acceptable to argue over whether "All Jews should be killed." A standpoint that suggests the outright destruction of an entire people, or their enslavement or removal of freedoms, is so heinous as to not even be worth discussing the possibility of merit.
First, only a subset of anti-semites believe or say that all Jews should be killed. I know that's a weird thing for me to say. Any kind of anti-semitism is of course despicable and irrational, even the lightest forms of it, but the distinction still needs to be made. It's much easier to talk to and potentially convince someone who merely believes in some common conspiracy theories and harbors low-level ill will towards Jews compared to someone with deep, zealous animosity and murderous intent towards some, most, or all Jews.
Second, as beyond disgusting an argument as that is, even such statements as "all Jews should be killed" must be (in a broad sense) "open for debate" if you truly believe in freedom of speech. If anti-semitism and other fringe ideologies can just be dismissed as banned topics, anti-semites (and far-right people who are against anti-semitism but find censorship to be anathema) will become more empowered, more secluded, more polarized, and potentially more dangerous.
Let's try to analyze what could lead a person to genuinely say or believe something like "all Jews should be killed". Let's assume they're being sincere, and aren't saying it just to be ironic or edgy or to piss people off. Many of these people have above average IQ and seem otherwise kind of normal. So how could this happen?
From what I've seen and experienced, the most common reason for this kind of sentiment is a delusional fear: they actually, truly believe that Jews are actively trying to gang up and exterminate them. Not just them in particular, but all Western Europeans/Americans/capitalists/right-wingers/white people or whatever their particular flavor of Nazism/fascism is. Of course, this is a ridiculous and false delusion, but in their mind it is a clear and present danger, and in their mind all of the evidence actually points this to being true in reality. They really believe they are in some kind of existential war with an organized "Eternal Jew" - a struggle for survival. And they believe that expelling or killing Jews is purely an act of self-defense.
Take a moment to just accept the craziness of it all. These people aren't playing with the same cards as us. They're fundamentally working off of a different impression of reality. They're on an unsupported fork of the "global/history" repo. All of their ideas and thoughts can be traced deterministically from initial delusional seeds that they accepted and integrated long ago.
I think the best explanation for most of these kinds of anti-semites is that they are a delusional/psychotic community of interrelated cult-like groups. And sure, there's generally no convincing crazy or delusional people, but you definitely aren't going to convince a, say, schizophrenic that he/she just needs help by decreeing "it is now illegal to think or say the things you are currently thinking and saying". (And no, I'm definitely not saying most anti-semites have schizophrenia or something, but I also would not be surprised if there's a disproportionate amount of overlap. I'm just using schizophrenia as an analogy.)
It's not easy, but racists and sexists can sometimes be convinced to soften their views or even change their stances. For example, Daryl Davis, a black man, managed to befriend many KKK members and leaders and convince them to completely rethink their entire ideology. (https://en.wikipedia.org/wiki/Daryl_Davis#Career_as_writer_a...) He treated them with decency, kindness, and an open mind (even though they did not deserve it), and many of them eventually came to treat him the same way and reject much of their ideology.
Or take the case of the son of the founder of Stormfront. Stormfront is (or was, before their domain registrar revoked their registration) one of the biggest and most fervent anti-semitic/racist/extreme-right communities out there. Derek Black, the founder's son, helped maintain the website until he ended up befriending some Jewish people in college. After merely being exposed to the fact that they and their families were kind, ordinary people for a few years, he renounced his racist views and dissociated himself from the website and his family. (https://www.washingtonpost.com/national/the-white-flight-of-...)
Both of these situations required years of exposure to opposing viewpoints presented in a respectful way to change their views.
They will never, ever be convinced to change their views if your response is solely to prevent them from discussing certain things or to ban them from places or even to advocate for (or ignore) violence towards them. (One might say "but they advocate for violence towards minorities", but as explained, that can be an oversimplification. And even when it is literally true, two wrongs don't make a right).
>My point: Restricting hate speech is not a slippery slope for freedom of speech. I personally believe there is no universal morality, but if I had to pick, I'd argue that the best outcome for the human race would be a culture that purges all racist and other arbitrarily prejudiced mindsets that judge entire populations on untenable grounds (race, gender, etc).
Again, we're entering dangerous territory. A private community/company, even one acting as a de facto common carrier of communication like reddit, certainly has the right to ban people who make heinous speech like this. But as a society, ideas like "we must purge all racists and arbitrarily prejudiced mindsets from our population" are untenable, unsustainable, and may be playing right into the other side's hands even aside from the slippery slope arguments and how exactly we should or could set clear boundaries of racism and prejudice. Even if we could theoretically set these clear boundaries (and we can't), this is still folly. The founding fathers and their predecessors in the Enlightenment knew this all too well. Freedom of speech really is an all or nothing thing. If something happens purely within the bounds of speech (e.g. the speech is not directly inciting or calling for a crime to occur), it must be permissible under the law.
I think this paper shows that this did help improve the overall balance of toxicity in reddit. But it does not (and pretty much cannot) show how this affects the greater ecosystem of hate speech online. No doubt the vast majority of the people who posted in those banned subreddits have been using many other websites and applications before the ban and are continuing to organize and communicate after the ban. They're just not doing it on reddit anymore.
And this is not something I would normally ever include in a comment, but since this is such a sensitive topic and since my comments here could plausibly be seen as some kind of tacit concern trolling that's secretly supporting anti-semitism or Nazism, I do feel it's relevant to mention that I'm Jewish myself and am extremely aware of the dangers and evils of anti-semitism and fascism.