Part of the reason why enslaved populations were considered to not be "on par" was often specifically about consciousness, intelligence, and capacity to feel, although sometimes this was expressed in roundabout terms such as "having no soul". For example, splitting families was justified on the basis that those mothers don't "actually" suffer as much as their owner would do in equivalent circumstances.

To be clear, I'm not claiming that the AIs that we have today are anywhere near the level where this is a real concern. But for something that can actually replace a human worker for a complex end-to-end task, that question will be much murkier. At some point we will inevitably cross the threshold where we will no longer be able to pretend that it does not apply without introducing metaphysical concepts such as "soul" to force a distinction.