its hard to see that its anything but censorship when these platforms hold a pure monopoly on the space. If Gutenberg prevented anything displeasing to him from being printed, we would rightly see that as censorship. Further, censorship takes place in many ways only a small number are the result of state actors. In Mexico reporting on the cartels will cause them to target and perhaps kill you.

If you want to argue that a good defense against censorship is to prevent a single entity from having too much power, I agree.

One is mandated, other is voluntary, but they both reflect the same phenomenon happening in the society - that more and more topics are becoming "not acceptable" and thus get eliminated from popular areas on-line.