My understanding is that post visibility is primarily a front-end issue - with some encouragement at the API level due to the eay certain calls work. For sure, an UI can ignore the APIs and do their own thing. So, agreed that censorship is primarily a front-end issue. There's some residual effect at the API level and, afaik, none at the blockchain level. Well, that is unless the witnesses allow it.
Thinking about that though, with the exception of a very small amount of immoral things, I think I'd insta-unvote a witness that supported blockchain level censorship. I realise that puts witnesses living in censorious jurisdiction under some pressure, but yeah, cross that bridge when it comes to it.