Well, while Slavery is historically heavily associated with discrimination atleast in the West, it's sort of different situations in the East.
As there wasn't really a concrete concept of Slavery in the East.
[ I might be wrong with what I say here as i am not an expert in history. ]
Most of the time it was more to do with Domination or Forced work (without restricting the personal independence of the person) than with Slavery.
What it meant was a person was forced to heavily work with really minimum wages (no wage work was really rare), but they were free to do whatever they wanted when not in work. People also used corporal punishment as a form of showcasing domination, but to local people that effect is bound to the person/people doing the dominantion rather than the system.
Like my origin country has a history of being under British rule for 90 years, and been under different invader rules for over hundreds of years before that. So in case of my history, you can say that the whole country was basically slaves to the invaders in a way.
And to most people here, Slaves wasn't a Concepts they they knew about, as the wording is used differently here (here the concept used meant "to serve", be it forced or consensual, and the concept of "to serve" is usually seen in high regards in here since ancient times so many did it consensually).
And most of the time, slaves weren't seen here, but rather people were sent from here to other parts of the world to work as slaves.
In all, the olden concept of Slavery in the East isn't that different from what we have with Modern Slavery in the form of Corporate Slaves, Forced Labor, Domestic Servitude, etc, which are usually ignored as they all come with monetary gain in some form and amount. So, to most Eastern people Slavery in a way still exists and in so common that it's part of life.
If I am right, there never was any Anti-slavery movement in the East either, as slavery as concept was abolished before long.
This is why, Japanese Author have no issue in using Slavery as a Concepts in series with medieval setting, nor do they try any Reforms system in the plot, as to most Author the concept meaning is different from what they know unlike how they understand in the West (only those Author who had read about Western history try to add reform movement within the story, like "Realist Hero" series for example).
Maybe this would give a better idea regarding what Japanese Author think about, than my rant (which might be full of errors).
https://en.m.wikipedia.org/wiki/Slavery_in_Japan
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