Don't tell people what kinds of roleplaying are okay for them. If they're having fun, it is presumptively okay, UNLESS it's exploitative, coercive, or insulting to the participants themselves. Is that good enough? Have I cleared your hurdles yet?
So, if someone posted that they recently had a game in which they played KKK members, and in the fiction they... did things the KKK was known for, or maybe they had a game in which they all tortured puppies and kittens, and everyone at the table was okay with it and they had an outstanding good time...
I think folks would be justified in telling them that was badwrongfun. We would get several requests to discipline them, or ban them from the boards. We have, in fact, banned accounts for less.
Now, you might give an argument that it was not the play itself that was bad, but speaking about it in public may cause harm to people who hear about it. But, really, if you can't talk about it in public without hurting someone... maybe it was a bad idea in the first place.