It doesn't seem to me like in TES Dunmer are the evil Dark Elf stereotype they are in so much other fiction.
Considering slavery was banned in Morrowind by the time of Oblivion, and it was a bit after that before things went boom boom... 200 years after it was abolished I see no reason for the Dunmer to all of a sudden try to have slaves in another province.
Look at how civil rights work in the real world. 150 years ago large parts of the USA were dominated by horrible people that owned slaves, and even many non-slave owners dishonorably defended this as a "state's rights" issue (I'm not making this a North/South thing or talking about the civil war in particular, there were riots in the North started by racist slavery supporters). 100 years ago women couldn't vote. It hasn't even been 100 years since segregation ended. 50 years from now people will look back at the civil rights battles of today with the same disbelief that we now look back at the time when women couldn't vote, or segregation, or interracial marriage was illegal, etc.
I think it would be boring if the world of TES was a never changing snapshot. This is different from my other post because my other post only justified it by using the Empire as an example. It has been 200 years for the clear moral value of banning slavery to spread and resonate. Unless Dunmer society continued to be contemptible so long later, it shouldn't happen and if it did it should be an illegal underground ring that most everyone in the province disdains and the authorities fight.