It’s also considered a word that is now owned by Black people, so when someone who isn’t Black says it, they do so knowing it isn’t a word that’s theirs to use.
And in America, expensive means valuable. Which also makes it the most expensive word to say. The point is, there are now social costs to saying or posting the word. Of course, no one is stopping anyone from saying or posting “nigger.” Instead, saying the word has consequences, like losing your job, your relationship or the respect of those who know you.
Probably around the same time people first learned they couldn’t say “nigger” in public spaces and forums anymore.