1: The chain only records text, so unless its ASCII art of child porn, I don't see it being an issue.
2: A permanent record like the chain is the last place somebody is going to want to share criminal conduct like that.
3: The chain is a database; so each front end can choose to show/not show any content they like.
4: In an absolute dire case, the witnesses can permanently delete something, like nuclear launch codes, but at least 17 of them have to agree that it needs to go.
I'm not aware of any examples of this happening.
I would oppose, and I'm confident almost all witnesses would oppose, any change to the chain that would let individuals permanently delete data.