For a while, I always figured that given how the Thalmor despise humans, it would make sence in my opinion that they practice slavery on humans. So I have it in my headcanon that there are humans, mostly Cyrodiilic, working in Aldmeri government plantations in Valenwood and Elsweyre, especially Valenwood. Though I doubt that there would be any in Alinor itself.

So how would they get the man-slaves? I'll list it.

1: Perhaps before the Great War, the Aldmeri Dominion would launch minor raids between the Cyrodiil/Dominion border. Small villages and minor Legion forts would probably be the main targets. Then the raiders would take what they got, captured villagers/Legionaires, and take them back to Valenwood/Elsweyre. And the Empire, being weak even even before the Great war, probably couldn't retaliate.

2: This one would probably stop number one. I would imagine that the Aldmeri had gotten a huge influex of slaves from the Great War. The majority of these slaves would probably be Legionaires unlucky enought to get captured; but many would also be civilians they took either before or during their withdraw from Cyrodiil.

3: Perhaps if the Thalmor caught a pesky Talos Worshiper near the Dominion/Cyrodiil border, they'd enslave them instead of killing them.

4: Since they got enough slaves, they probably breed them. So some of the human slaves are the sons and daughters of the ones that had freedom before.

So I ask you all this question, is there any source that contradicts this?

I dont think there would be any slaves, just prisoners of war that are worked to death (like how many Snow Elves were in Skyrim, they built Windhelm after all)