I would not write Elves as a slave-race. It just doesn't seem fitting. Individual elves as slaves works though. It is a bit silly if you have an elf that's been the slave in a noble family for generations though. (At least one manga did this and the MC points out that the elf could have broken free at any moment she wanted, so she was probably into this and took a "vacation" pretending to be a slave for a century.)
So in my erotic fantasy stories, there is a sort of theme or trope that carries between settings.
Y'all remember in The Bible how God gives mankind Dominion over the world and all of the beasts within it?
Well, humans in my stories can use divine magic to force non-humans to believe God is watching them and judging them. Those who are so magically bound cannot help but feel like if they fail in their duties as slaves, then they will go to Hell. The slaves are kept in line by their own guilt and shame.
The humans take it for granted that the psychological impacts of the slave codes being imposed upon their chattels' minds as heavenly mandates compels the tamed monsters to zealously serve, obey, and please their human owners.
There are no "slave races" in my fairy stories, but there are monsters that can be tamed, trained, and domesticated.
The issue is, do humans believe elves ought to be be under their dominion like all the other monsters or do humans believe that elves are special and that God placed the beautiful long-lived race of torture junkies as mankind's equals or superiors?
Do humans think that they can use the same spells that compel orcs and furries to work their fields to make an elf want to warm their bed?
Would the spells work or would the elves laugh off human attempts to enthrall their souls?
Strongly disagree! Captivity is habit-forming. Remember Malcolm X's speech?
And that's in the context of the worst kind of slavery, chattel slavery. There have been less horrible types of slavery throughout history, for example in the Ottoman Empire, in which slaves could reach high rank.
Also, historically, many noble-born children were raised by servants and slaves, and it was common for them to develop a real parent/child relationship well into adulthood. I could definitely see a story in which an Elven slave raised the children of one particular family for generations on end.
And that is in the mundane world with humans enslaving other humans without magic.
Add magic, interspecies competition, and the a multiverse of realms from which to plunder, and calculus of cruelty changes.
Regardless of whether humans can bind elves or not, the elves will absolutely use their longevity to exploit the naivete of the young and the nostalgia of the old to make humans see them in the best of lights.
Even when they are slaves, the elves are running the house. Like Steven in Django, they are the power behind the throne.
The one race so far that I have as immune to soul-binding magics are Dwarves.
Dwarves know exactly who they are, who made them, and why they exist. Dwarves cannot be bound by human magics because they are bound to something much, much stronger.
Humans do not mind this since it means that Dwarves are reliable business partners with access to a transcosmic trade network.