In Civs 3-5, workers lived on, the same as most military units. Capturing workers in battle made sense; one could even *trade* for workers in Civ3. In Civ1 and Civ2, workers didn't exist; settlers served to both found cities and carry out terrain improvements. Several of those games allowed workers (either captured or native) to be joined to cities to increase their population.
Modeling an enslaved worker in the Civ6 system, where builders have a finite number of charges, doesn't make as much sense to me. In the normal process of the game, I use production (or gold or faith) to generate a unit that DISAPPEARS after its work is done! The main reason that so many groups engaged in slavery (as
@Boris Gudenuf described in post #2) was to keep the captives around!
For me to consider either a) a return to "pop rushing" from earlier civ games or b) representing some sort of involuntary servitude, we would need to move back to workers instead of limited charge builders.