Most authors have self censored for a very long time, otherwise they don't sell books. Jules Verne famously changed the nationality of Captain Nemo because his publisher didn't want to offend various foreign governments. There are, of course, some authors who like to be controversial. Social media does make it al lot easier for those who find something offensive to stir up outrage. Before social media, you needed a certain level of connections and influence to do so. Now, anyone can do it.
Being an author is no different than most small businesses, you don't offend the customers if you want to stay in business. You can call it self censorship, or you can call it knowing the market place. Now, if by self censor, you mean authors avoid certain topics or characters because they worry a small extremely vocal group of people will stir up a tweeter storm over, then I'm sure that's the case.
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