I'd be interested to see if the author bothered to look into different systems of slavery. For one it is not correct to say that systems of slavery didn't persist in Europe. You had some straight up slavery during the entire period. Not to mention things like Serfdom which often was slavery in all but name with Serfs tied to the land and their lord and who could be bought, sold, and mortgaged like other property. Indentured servitude was common and whilst that had a time limit the indentured servant was not free while they paid down their debt.
If we take the tack that this is pre-paper Europe (so pre 11th-12th century or so) then slavery is still reasonably common with, for example, 10% of people recorded in the Domesday Book being slaves. The taking of Muslim as slaves was common into the 15th century after which focus switched to African slaves with the use of slaves more common in Iberia than some other parts of Europe. Likewise Christian slaves were taken by the Muslim powers and pagan slaves (many of the Slavs and the precise entomological link between the two words, if any, is disputed) were widely traded between Christian and Muslim powers.
As the various Crusades petered out and Europe was mostly Christianised the supply of Muslim and Pagan slaves dried up which then shifted the focus to Africa. It was a source of slaves to Iberia before the triangle trade opened up and the awful conditions of chattel slavery in the Americas became a voracious eater of humanity consuming as many slaves as could be supplied in its inhumanity. That is all significantly after the 12th Century.
A side bar to all this was Christian Thralls in Scandinavia who were around until Scandinavia Christianized and thralldom was essentially outlawed around the 14th century.
Side bar two, the first broad restrictions on the slave trades where imposed on the selling of slaves to Muslims by the European powers and vice versa. One reason for this is that these slaves were deployed in war (the Ottoman Janissaries for example were made up mostly of Christian slaves converted to Islam). The Barbary Pirates took up the slack. They raided for slaves around the Mediterranean into the 19th century, though their heyday was in the 16th and 17th century.
I should add that I think that slavery should be universally reviled but that even then you have a wide variety of systems some significantly worse than others with chattel slavery in the Americas amongst the very worst. I also find it very distasteful when slavery is just stuck into a fantasy story since the author has a choice in how they built their world and most handle it badly. I would especially question the morals of any author who stick a slavery system in their world if it is mostly used as a supply of women for the protagonist.
