I just permanently put down a book–sadly, no, this is not an “I can in fact quit you” post–because the historian writing it was taking every single period usage of a particular insult as though it was documented fact. He didn’t seem to have realized what the other historians of that period had: namely, that while some behaviors probably were well-supported, a single mention of a political enemy calling someone a witch, a sodomite, a cripple, or some other term assumed by contemporaries to be negative did not mean that they engaged in any particular concrete behavior whatsoever–except to disagree with the political enemy.
We’ve had this in our own lifetimes. Of course we have. People get called gay, Muslims, Communists, terrorists, because the people using those terms think that those are negative things to be, not because they saw someone kissing someone of the same sex or praying to Mecca, much less anything that might in real terms constitute affiliation with any Communist (or communist) or terrorist group. And yet historians! Come on, historians! This is supposed to be your job! This kind of perspective: you’re supposed to know better!
But in terms of writing speculative fiction, I think this kind of culturally pervasive insult can rise above Princess Leia’s “scruffy-looking nerf-herder!” if it’s handled carefully, but only then, and it’s pretty tricky for exactly the reasons the historians had trouble with it. One of the main ways we learn things about characters in a novel is what the other characters say about them, so if Ana says that Bot is a filthy drum-sniffer–what’s a drum-sniffer? why shouldn’t you sniff drums? what do they smell like?–we, the readers, have to find out that there’s some reason not to believe her, or else we do go away with the thought that, well, that Bot, he’s a filthy drum-sniffer. And we have to know whether drum-sniffing is an actual thing that carries with it serious shame, whether it used to but has fallen into slang usage that no longer feels literal (as with “bastard” no longer carrying serious allegations of parental non-marriage), and so on. Whether something is potentially a literal truth and only some people find it insulting is one of the hardest ones to pull off–the speculative world equivalent of allegations that President Obama is a secret Muslim, for example. It’s hard enough to navigate the thickets of “He isn’t, but it wouldn’t be an insult if he was” in this world, where there is an actual President Obama whose external religious behavior can be observed, and where people can look up external definitions of what “Christian” and “Muslim” mean to various parties.
It’s another piece of worldbuilding that can add richness and depth to the culture(s) and personalities you’re building, or it can bog a story down and confuse readers needlessly. If it happens to actual historians, it can certainly happen to fantasy readers. But that’s not a reason not to try, it’s a reason to be careful and run things past test-readers. Like most of the things worldbuilding nerds talk about wanting to see more of, it shouldn’t be required in every story, as a checklist, just as one of the cultural touchstones that can spark implications and ideas.