When did ‘negro’ devolve into a slur? I realize that what used to be the polite way to refer to people whose immediate ancestors came from Africa is now considered old fashioned, but a slur? As has been pointed out, it is merely Spanish for ‘black’.
Here’s my best guess. For the most part the word has fallen out of usage but it’s close enough to that other N word that remaining vestiges have to go in the so called minds of the woke.