@4V29LN0n
You make a lot of assumptions on a few things:
1- Slavery is not "bad". Slavery has a lot of negative connotations and is, in general, not something good to have anywhere but that is because it cause a lot of economical problems for the society that has it and not for any moral reason, though there are moral reasons for not having slavery as well.
2- Slavery is a necessity for low technological societies. For more that there are negative aspects to slavery it also has positive ones and some others that are not good nor bad but cause slavery to be obligatory for those societies.
*Those two are obvious for anyone with even minimal knowledge of history.
3- It would be unrealistic and a loss of opportunity to not have slavery in the setting as it opens a lot of possibilities for the story.
4- Your problem is the mere existence of slavery in the story, just like the people that get all bothered about fan-service, and it doesn't matter how it is implemented you will always hold that same opinion, which is a stupid thing to do. Slavery is usually portrayed in Isekai as something that exist and the way the MC interact with it are 3: They don't like it and do what they can to stop it from existing, for some reason end up getting slaves themselves and treating them as companions instead of slaves or get slaves to use as slaves. In the first two situations there is very little to complain about it and the last one is usually for when the MC is not someone that should be liked by the reader or are some "bad person" type character and there are very little problems with all three ways. I would understand it if this were some story that romanticize slavery, like some stories do where the slave is willing to be a slave and does so because of some romantic reason like in Tate no Yuusha no Nariagari, but even that kind of representation of slavery is not unrealistic.
