Slavery is not very "fantasy" isn't it?
Slavery is exceedingly fantasy related even if it also exists in reality. Everything from mind control to overly powerful individuals seeing themselves as above others, widening the normal gap in power structure between the wealthy and poor even more.
Slavery is par for the course in any kind of remotely realistic fantasy world, people pretending it's not realistic are just shutting their eyes while pretending to be blind because of PC. But humanity is simply fond of the idea of feeling superior over others and proving that they are worth more, just because people suppress it in today's society doesn't mean the desires aren't there anymore. We're just intelligent and sociable enough to know that it's in our best interest to not act like morons, but what if people had supernatural powers and there was no proper government to rein people in tightly (or that said government wasn't able to because some people are too OP)? Of course slavery, discrimination, class structures, inequality and so on would reign. This is more true in a fantasy world than one ruled by typical science.
Of course, the portrayal in isekai is often rather unrealistic even if slavery often exists, it's typically just used as a crutch for a virgin/inept protagonist to get a cute girl or two easily that are completely devoted to them. Simply a fetish thing instead of true slavery and that's fine, really. I can't be bothered with people complaining about slavery, rape, murder, lolis and other controversial issues in what is essentially just self-insert stories where no one was actually hurt. It was a long time since I stopped caring morals with mere stories, let people enjoy their fetishes, everyone's happier that way in the end
