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.