This requires nuance and discussion, but things like the following are generally not productive and not protected by free speech.
Defamation against private individual (including libel and slander)
Child pornography
Blackmail
Incitement to imminent violence
Solicitations to commit crimes
I think any discussion concerning "ideas" is fair game. It gets murky when dealing with individuals. None of the things on the list above are an "idea."
I admit I was playing devil's advocate, but I appreciate the good faith response.
I genuinely do have trouble unifying my instinct that nazi propaganda, for example, should be censored, against my understanding that what constitutes "solicitation to commit crimes" is relative to what a society considers "good" or "bad," which is (somewhat) arbitrary.
Take your list as an example. Any action that is damaging to the State could be considered Criminal - perhaps something as simple as developing good cryptography, for example. Some things are crimes that shouldn't be - smoking marijuana, an example. Simply saying "solicitations to commit crimes should be censored" thus doesn't jive with me.
Yes I think it is impossible to completely codify a list. I can always think of exceptions, grey areas, ways to exploit individuals, and ways individuals can create "loop holes."
I think the main problem is the human heart ... but that is a different discussion.
For example: claiming gay people are pedophiles, or that trans women are men in dresses trying to sneak into bathrooms to assault people.
Claims like this--and these are the tip of the iceberg--target groups of people and affect every individual within. It's like passive libel/slander that every member of the group has to contend with.
It can even affect people outside the group. I don't think I know of a straight man under 30 who isn't afraid someone will think they're gay if they do the wrong thing. And more than a few cisgender women get run out of bathrooms because they present in a more masculine way.
Publicly denouncing a group of specific folks as communists, when they were in fact not communists I believe was illegal even in the McCarthy era (that's libel if posted and slander if spoken publicly).