Clarification, what I am about to talk about is NOT American slavery, but the slavery that came before, which wasn't purely race based.
The thing I don't understand is why people freak out over the introduction (or, rather, preexistence) of slavery in these medieval (or even premedieval) civilizations. What are they supposed to do with their criminals, their debtors, their orphans, etc? Let them die? They don't have enough food or security (of life) to put them in jail or prison, letting them go would not solve the crime issue (and would kikl thr orphans anyway). What other choice do they have? (Although, I won't argue that sometimes death is the better option, but sometimes it isn't).
Slavery is wrong, obviously, but the American prison system (and certainly the death penalty) has also (quite often) been argued to be wrong. Locking someone up in a box big enough to hold a bed, a toilet and a desk (if you're lucky), with no possible way to leave (early, I am referring to rehabilitation methods), around other people who are just as likely to kill you as interact with you (if they think they can get away with it). How is this better than working your debt off through hard (if forced) labor... Which we actually have the technology to do at this point.
No. What those traffickers were doing is horrible as it is stated (suggested) that the elves were captured, rather than being punished. But the society that doesn't work to resolve it - due to fear of irrational prosecution - is worse for literally ignoring the crime and those asking for help. Hell, kill the traffickers and release the captured people. If they were judged and found guilty, then sentenced those transporting them shouldn't have to hide it.
(B.t.w. I am not going to comment on the judicial system, I don't think we have a stable enough system for me to go onto that here.)
There is one Isekai that I think did penal labor well. I cannot remember the name of it, but the people housing and leasing (as it was more a leasing system than a selling system, unless the individual agrees to the transfer of responsibility) were actually responsible for the health and welfare of the individuals they held. These individuals still had a debt and were therefore not free, but they were protected.
Problem is, no real world would adopt that system without abundance of food, land and resources... Which is decidedly NOT any Isekai world...