@Rei I think you're missing the point. The author didn't add slavery because they support it—they added it because they're portraying a problematic historical period. Fiction can show terrible things without endorsing them. By that logic, a story can't include war, racism, slavery, or abuse unless the author agrees with those things, which doesn't make sense.
Also, it's a bit inconsistent to criticize Song of the Wasteland for depicting slavery while having sua from Alien Stage as your profile picture. Alien Stage is literally built around a system where humans are treated as property and forced to participate under alien control. If showing a master–slave relationship is automatically wrong, then wouldn't you have to dislike Alien Stage too?
It's fine not to like slavery being included in a story, but saying a work shouldn't depict historical oppression at all while enjoying another series that uses similar themes feels hypocritical.
I have read way too many yaoi manhwa
I'm a girl