It depends on the context. Friends of different races ragging on each other about race or ethnicity is fine, so long as they realize nothing being said is serious. I had a friend in the military, who was Native American, tell me he would, “slap the white off my face.” I don’t remember what the conversation was about, but I remember that particular insult because it was so weird and I couldn’t stop laughing. That’s very typical of the kinds of things you’d hear people say to each other in the military, and it went on all the time. People who think otherwise simply don’t understand how guys in their late teens and early twenties, and who in the the military, talk to each other.
I can easily imagine someone jokingly telling a black friend, “Go home n*****.” and everyone laughing. The problem with an insult on a whiteboard is that you can’t tell if it was a genuine threat or a joke. And I still wouldn’t be surprised to learn that the supposed targets of the insult were the ones who wrote it on the whiteboards. That kind of thing has happened before. People who are absolutely convinced that this was a threat are jumping the gun.
I was in the Air Force in the late 70s-early 80s, but I’m not buying into the idea that what was acceptable to say then isn’t acceptable now. It’s just that people have become absurdly hypersensitive about it.