The irony about the slavery tag is that we essentially already have it in the form of sentients like Tigermen that live in our forts as "pets".
Therefore, as an ethical term relative to gameplay, slavery is meaningless in dwarf fortress.
But it's not, as tigermen and other such pets which also happen to be sentients have never worked properly, and probably won't ever work properly until the issue of them being part of a foreign civ is addressed (whether that itself is relevant to slavery or something else entirely, like non-player-civ migrants or whatever).
I'm not arguing for slaves in the conventionally understood human sense of the term.
Even that doesn't work, because dwarfs aren't a species that are morally inclined to hold slaves.
And I think the implication goes beyond the "right and wrong" of the act of enslaving another sentient species, also. I suspect that our dwarfs would find it distasteful to allow another species to do "perfectly good work a perfectly good dwarf could do."
The real larger issue is the functionality applied to non-fort sentient beings, which is non-existent to the point of brokenness.
Examples of this are that sentients like Tigermen pets in the fort die of thirst quickly but have no way to drink water, or when trained as war animals/adopted as pets, follow their masters blindly to the point of starvation. Beyond that, we also have issues with loyalty cascades.
I'm merely trying to justify the enabling of functionality for sentients. In this way, we can avoid all the flavor arguments that surround the issue and only serve to muddy it up.
Allowing functions for sentients doesn't have to entail chain-gangs of goblins hauling rocks to your megaproject. It can start as simply as creating that recognition of non-player civs in the fortress, which 3 refers to.
This has many applications that would not be forbidden as slavery. For example we might be able to allow Tigermen "pets" to be assigned to dwarves as guardians and wield weapons while at the same time not causing them to die of thirst because they can't stop following their masters or having their war training break their sentience (both of these are bugs in the current game). Maybe tame werewolves could be enabled to do very basic things like bring water to thirsty dwarves or rescue crippled soldiers, thus acting as always on-call labor for dangerous or low-level tasks you wouldn't want to risk or waste valuable dwarves on.
Non-player race sentient functionality would also cover rebellions and uprisings, so sentient races bred at your fort might rebel if mistreated (leading to Fun!), and this could even be extended to dwarves themselves, allowing a faction of dwarves to break away and form a group of insurgents hostile to the player.
Other things this would cover would be allowing Dwarves to bring war animals or mounts, allowing the player to replicate the invasion forces of goblins mounted on beak dogs with trolls or humans mounted on War Lions accompanied by trained Giant Eagles.
These things are not mere bloats; they are part of the functionality eventually intended to be in the game.
After this functionality has been added to the game, slavery would be child's play to mod in. We don't have to add slavery on its own, but clearly it can be added incidentally as part of a wider number of changes that would make the game better and remove a lot of the bugginess and non-functionality that we presently experience.