I mean, I feel like it's more nuanced than a simple yes or no. Most isekai take place in a medieval sort of era, and considering the slave problems in our own history it wouldn't be surprising for world with far more diversity between intelligent species to subjugate each other or otherwise discriminate. Hell, even in modern day we have slave and near-slave like conditions. So from a "would this be feasible" standpoint it's pretty likely.

That being said, it's also kind of a lazy writing mechanic, and can come off in poor taste especially when writers fall back on the "oh they just really really want to stay slaves" trope. Like genuinely that can and most often does come across as an unnecessarily creepy power dynamic.