I wouldnt mind slavery being part of (some) fantasy series as like you said its sad fact of history. I just wished it would get treated with seriousness it deserves. Im not expecting all series to turn into documentaries about cruelty of slaverty or mc:s to start slave revolts. However at very least i wished the constant fetishization female (often sex-)slavery would stop (honestly its also really overused and boring trope at this point). I especially hate how many series go out of their way to try to make a point about how "slavery is actually good in this world".
You do know that not all slaves in history are treated poorly. Yes many were, and over all it was a is a terrible thing. But remember there are many types of it in history. Most associate slavery to what happened with Africans. But that is really such a small fraction of it in history. Like in Rome and Greece. Captured warriors could rise up the ranks, each year they rose up social status wise starting as slaves but many of them eared citizenship. Even the colosseums was very different than many are taught. It was far from the bloodstained killing grounds its remembered as. Criminals were often killed in it. But the slaves which were often captured enemy warriors were actually well taken care of, and very rarely killed in the fights.
This comes down to one the key thing so many forget, the owner is investing in the slave. Them being in bad shape, and worse dying is nothing but a huge loss. Most slave owner in history actually made sure to keep them fed, clothed and with shelter, them being in good shape was preferable. And this is where we see a the big gap. The cases history tends to talk about are where it was a major land owner, noble or some other highly wealthy person. People like them who can afford to replace slaves where the ones who treated them poorly. But the majority of owners only had one or two slaves at most, they would often be raised alongside the owners kid as a companion. Or be a nanny or help around the house. In many cases they were actually very close with the owners and treated extremely well.
Its far from good, but in many cases it was better for them than their other options. Cases like small villages selling a mouth so the rest can eat for the winter, leading to both the family living, and often the one sold was given a job and taught the skills needed for it. It was often part of basically apprenticeship systems, where the workers are taught the skills of a trade and in exchange they enter into a type of slavery till they master the craft. Maids for instance were almost always slaves owned by nobles, but in many ways their social standing was actually higher than before they were bought.
Now I am not at all saying this is a good thing, or that slavery is something to praise. But my point is that most of the horrible aspects, are not really tied to slavery but other problems.
Poverty and starvation leading to no better options, which is more tied to greed, corruption and countries pride getting in the way of food getting to where its needed.
Lack of social safety nets, healthcare, disability and the like. These are getting better, though still need work.
Education, its a lot of time and money to teach people, and in the past learning to read or special skills was not something many could afford. So in order to learn they had to take on debt and become a apprentice.
There are many things we take for granted now that make us not think about what benefits a slave had from slavery in the past. As many of those things have become basic rights now. But in so many cases, becoming a slave, was a means to survive. After all staying alive back then was rough.