There’s actually a god reason to say “the n-word,” instead of “n!gger,” in the US. Saying the actual word can come back to haunt you.
Mark Fuhrman apparently had at one time in his life, a few decades before the OJ Simpson trial, actually used the word to refer to black people, but at some point in the far past had exorcised the word from hi everyday vocabulary. I believe it was at the strongly worded suggestion of a superior, but he nonetheless did it.
Then three or four years before the trial, he was interviewed for a book or article (I’ve forgotten which) by a writer of some ilk about racism in the police department, and he went whole megillah describing the sort of language other police officers use-- both content and specific vocabulary.
He may have been enjoying himself a little too much, but that’s neither here nor there-- he chose not to use euphemisms, and said “n!gger” a WHOLE lot. Also, apparently, “kike,” “spic,” “wetback,” and who knows what else-- Chink, Gook? “he Jewed me down?” I don’t have transcripts in front of me.
What happened was that he said on the stand, under oath, that he had not said “n!gger” for over a decade, as evidence that he was not a racist. To impeach him at trial, the defense brought in the writer, and her tapes of Fuhrman frothing off all sorts of racial slurs, including “n!gger” again and again. And she had exact dates.
Now, in one sense, Fuhrman hadn’t lied, because he hadn’t called anyone a “n!gger” for a very long time. But in strict point of fact, the word had passed his lips, and that was what the jury heard.
White people watching that trial learned that it wasn’t a good idea to used the word no matter what the context.
I can’t say for certain that the use of “the n-word” to replace the actual word in discussions of the word goes squarely back to the trial, but my sense it that it does. It goes back to the late 1990s, which means right around the time of the trial.
There was already a pretty big movement not to use the word-- Huck Finn was being taken out of lots of school libraries, and off of school reading lists, but subbing something else in, like saying “HaShem” for “God,” or “He who must not be named” for Voldemort, I really think goes back to the Simpson trial. No one wanted a paper (or video) trail of having used the word.