Slavery is like an old disease such as Polio: it still exists in some part of the world, but we're progressively eradicating it.
Looking at how societies trended away from slavery, it might just have been a local optimum at some point in time, but only by accident: autonomous agents seem to deliver more output by having more creativity when they're free to explore the alternatives
Even leaving aside the benevolence that sentient being may have for other sentient beings (because having more friends is having more fun!), whether it's humans or AI deciding, I don't think there's a good case in the long run for one putting the other into slavery.