I know it's a common ingredient in isekai stories, but I'm still put off by how normalized the depiction of slavery is. It's just a morally-neutral part of the isekai like experience points and magic. Are there any isekai protagonists out there who are at least squicked out by the idea of buying and selling people like livestock?
Yes, quite a few. Even this FMC doesn't like it but has to use it.
Skeletal Knight is another isekai that slavery is portrayed as taboo and it's actually used for nefarious means.
There's also another one that I can't remember the name of that the premise is that he has a skill to break the slave collars and just goes around gathering a harem using that skill.
I think it gets used so often in isekai because they are portraying life in most cases pre-industrial eras which slavery/indentured servitude is very common. The fact that the MCs are okay with it is another thing but in their shoes they are getting used to so many differences that they probably are just getting information overload.
Ultimately, slavery is used as a lazy writing shortcut to instantly bond characters together without having to develop their relationship organically through actual storytelling.