It cuts both ways though, you can't have your cake and eat it too. A counter example: if I'm the only guy I know that thinks Jim Crow laws are bad and there's nobody around to buck me up, so maybe I consider that there's something wrong with my beliefs.
If you police speech for some people and not others you will create problems, either you think that people are inherently good (therefore everbody should be able to speak until they reveal otherwise) or some kind of top down censorship should be in place to keep things "civil", now the problem is who gets to decide what "civil" is.
So its better and easier and more robust to just let people say what they want to say (even if bad) and just allow people to filter out what they don't want.
Further, "collecting" a group of scattered folks with common interests can be good even if their common interest isn't. E.g. I know that there are "cutting" and self-harm websites, and of course the practice of self-harm is awful, but those websites may be a way to reach people and help them.
As to what to regulate, I think that the anti-Holocaust denial laws in Europe make a more fraught case than anti-child pornography. Child pornography generally hurts kids even if it's not circulated.
Denying the Holocaust is stupid and offensive and upsetting to many, but it doesn't harm the speaker or the listener in the same way. I don't know, one way or another, whether these laws have led to "slippery slopes".
It does cut both ways, but that doesn't justify your "let people say what they want no matter how awful it is" conclusion.
At the margins it's certainly hard to tell sort-of-bad content from not-so-bad content. But that doesn't mean we can't find pretty clear examples. Banning child pornography, for example, doesn't seem to have harmed freedom of speech much.
You also ignore that what speech we allow influences what people will say at all. For example, if we allow violent threats and harassment (which are both definitely speech), this will be used to silence voices. And in the American context, it's pretty clear which voices get silenced. Which voices have been silenced for decades and decades.
So in practice, somebody will always be silenced on some topics. The question is more who gets silenced, and what kind of environment for discussion we want to create.
I don't think that child pornography falls under speech at all, and I don't see the connection. Child pornography sole purpose is to serve prurient interests.
Sorry I don't mean to say that speech should be a free for all, threats will not be tolerated, nor would harassment, but we already have longstanding laws and procedures to deal with those things, they are nothing new.
"it's pretty clear which voices get silenced. Which voices have been silenced for decades and decades"
I don't think it's clear at all, nobody is currently being silenced, however I can see the pattern of existing laws and policies around harassment and threats being misused to silence political opponents. This is a very bad development, and mostly recent phenomenon. I think as far as thats concerned we have crossed the rubicon so to speak, and the polarization will only accelerate.
"So in practice, somebody will always be silenced on some topics."
I don't agree. Even child pornography can be discussed however children have no agency and can't consent to their images being used in such a way. So it can never be legalized. There are many subjects that cannot be discussed (racial issues, social issues) that are considered by the left to be unmentionable even though points expressed are perfectly respectful and salient.
The US has a long track record of violence, threat of violence, and social power being used to suppress women and black people, for example. The #MeToo campaign happened recently not because women weren't being sexually exploited in the past, but because their power has finally grown enough that some of them can start to speak with less fear of consequence.
Studies show that this power imbalance continues online. Accounts with female and/or black get a lot more crap. Crap that acts to suppress not just the people receiving it, but anybody who sees it happen and thinks they might become a target of it.
So if you're running a platform, you have a pretty clear choice. Somebody's getting silenced. The question is whether you let the people who want to silence others run rampant, or whether you protect the voices they're trying to suppress.
And I'd add that the left talks about racial and social issues quite a bit, so I think your characterization there is at best misleading.
If you police speech for some people and not others you will create problems, either you think that people are inherently good (therefore everbody should be able to speak until they reveal otherwise) or some kind of top down censorship should be in place to keep things "civil", now the problem is who gets to decide what "civil" is.
So its better and easier and more robust to just let people say what they want to say (even if bad) and just allow people to filter out what they don't want.