Who is in charge of what’s deemed “harmful”? How do they decide? Who put them in charge?

The only practical answer is that users should be able to decide for themselves.

Anything else just devolves into government or corporate censorship.

There’s no such thing as corporate censorship. That’s largely been manufactured from some who have a persecution complex. The only way corporations can censor someone is if that person is accessing property or platforms that the corporation owns. At which point, they have freedom to do what they please when it comes to who they will host. That would be like saying your neighbor is censoring you because they won’t let you on their property or use their things. They can’t legally do anything more than remove you or deny you access to things they operate. The government censorship is a logically real thing in that they have the power to create laws that affect you regardless of property/object ownership

Aye corporations can do no wrong, they’ve never tried to censor unions or the working class.