Before social media, it was normally considered impolite to discuss religion, politics, or other sensitive topics in the presence of people you don't know well. I think this is probably good advice on social media, though people forget it.

I don't think it's self-censorship so much as a thing humans do to maintain the fragile network of relationships that holds society together. There are all kinds of things we want to do but refrain from doing in order to be polite: is not picking your nose in public self-censorship?