As a proud Roleplay and Real life racist,
I think it's just important when we have discussions like this not to deprive ourselves of context. When talking about OOC conduct and intent of interfacing with server, it's not a light matter to assign thoughts to people they don't express themselves.
I don't particularly find myself to be friends with people who are reported for using slurs in Roleplay, but I do find myself sympathetic to them because it is a roleplay server- it is actually a GOOD quality to be willing to roleplay a character with brazen flaws. I think the a good way to combat people using it as an outlet to field genuine IRL malice is to just create the social expectation that characters with these brazen flaws are not celebrated OOCly for those flaws.
I'll offer my own experience as an example; Norli Starbreaker is a racist. Not just in the @sshole way of using strong words against people in a lack of sympathy to their emotions, but in the genuine believes in racial hierarchy way- and in a very cruel subtle way as well where he will speak kindly to groups he thinks lesser of. As I became a more veteran roleplayer, I found the best way to deal with that was to call attention to it in the description/action segment of any emote I did. When describing how he says something, describe it as arrogant and flawed.
I won't pretend that this is alone is an antidote to dealing with this subject, but I do think it's a good way to deal with playing a character with the flaws- make the fact that they're flaws part of the character.
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As for the specific word "Darkeh", I do just want to say that's definately something used a lot within the dwarves both for dwarves and outsiders. I saw someone comment "but dwarves are black too" and I will reply to them that yes and they call eachother that as well. I think it's worth being cognizant of how/why people use slurs, or at least not over simplifying it. Don't deprive yourself of context- if your position requires you ignore context then it's a bad position.
I will genuinely say I've never heard it used at all in real life, and I've definitely heard my share of demeaning words in real life, so I really can't speak to the impact of it IRL. I will observe however that if it is about the same level of prevalence or less than the term "spook" then we should probably permit its use. However I do genuinely feel that I have no awareness of its prevalance, and even in trying to find out more from researching I couldn't find anything.