Slavery can be "okay" if the story fully embraces the fantasy and never tries to be anything more than fetish bait. Pic related set itself to be just about the MC doing lewd things with his slaves and dungeon autismo, nothing more.

But some authors go full retard and try to have the MC being strong enough to topple kingdoms, but somehow the All-powerful MC can't stop slavery because....pick any bullshit reason. In real life slave rebellions were a constant thing, escaped slaves building their own hidden communities also happened constantly, some guys had the job of just hunting and destroying those communities in order to stop any slave from getting ideas. Thousands of slaves would rally and fight for your average isekai MC, just for the promise of freedom.

Rome would have dropped slavery instantly if some powerful unstoppable guy was going around putting every slave owner on a cross. America would have never embraced slavery if some mage was going around turning slaves owners and their families into burning pyres. Plenty of Isekai MC's have more power than that, but they refuse to do anything to stop slavery....because the author wants his story to be "serious stuff" and not just softcore porn like Isekai Meikyuu. Sorry, pick your lane and stick to it, either embrace the softcore porn route, or make your MC have enough dignity to not engage in slavery.