Y'know, I never thought about it, but I actually don't think you can outright enslave anyone. There's some stuff that's very slavery-adjacent, but it's never explicitly slavery.
Alissa, once captured, is a prisoner, but you can sexually exploit her to your heart's content, so I guess she counts as a sex slave.
Same goes for Asgeir and the Corrupted Lupines if you choose to imprison them, though their content is a LOT less exploitative than Alissa's. I guess they're more "prisoners that are up for sex" than slaves.
Jae'lyn is technically a slave, as she's stuck with you and has to obey your commands, but she does also get an actual job and seems to more or less enjoy herself.
You can choose to take the Catgirl Harem from Hirrud during Missing Meows. They agree to be yours consensually, but their position is incredibly compromised and you hold a lot of power over them, so it feels a bit like slavery even though it isn't.
Speaking of that quest, Hirrud Grune can be TF'd with Bimbo Brew in Khor'minos and sent to the Wayfort. At that point she's there willingly, but I would consider it bordering on slavery. Especially because you can assign her to be stress relief for the soldiers.
That's all that comes to mind.