I've largely found MUSHes to be far more progressive when it comes to gender/sexuality politics than when it comes to race. I think largely because MUSH population is predominantly white, and that gender and sexuality issues are issues many have faced. Not so much race. Not really at all race. I know, being as European white-bred as I can be, I sure haven't. It's very alien to me, even though I've done all I can to try to learn and educate myself to the experiences of others.
Even then, whitey ass me still gets surprised!
A lot of players I've found -- generally those in the ok boomer-to-gen X age range -- haven't really updated themselves out of old mentalities, and will surprise me with old, non-PC terms (mulatto or Oriental I've seen many times, and was surprised by it) or ignorant prejudices that if they're not saying or doing anything hostile or negative toward a race, then they're not racist. Not all players in that age range, no. But it correlates.
There are also a lot of players I find that like to cling to the older tropes of game types, and elevate "normalized geekery" over what may be considered offensive. I've witnessed superhero games that keep FCs from the 1940s that my be built on or simply ARE racist cariactures ('The Mandarin', anyone? Or 'Gypsy Moth'). I've witnessed fantasy games that like to keep that Tolkien-esque golden age of racism where the evil overseas empires and races are usually going to be dark-skinned in some way.
For a recent example, I know just in Arx, I've felt many times uncomfortable just with the occasional jokey player culture of 'unleashing a crusade' on the made-up Arabic-themed empire. Even though I'm certain it's not theme trying to promote such a thing, at times I can witness a bit of slippery slope happening with how some of the playerbase receives it, and can be a little bit Too Real to be able to keep it safely in the realm of the fictional.
I think it takes a lot of care that people may not always employ, because they're too busy having fun geeking out to what feels normal and fun to your average middle-class white nerd, and sometimes having to see that Normal Geeky Fun in a different light -- potentially, an offensive light -- isn't everyone's cup of tea.