Nah that’s different. For humans it was an effect of might is right. Slavery was there for cheap labor. It still had a cost since you had to feed them. And many times slaves bought them selves out of slavery.

Today you still have slaves as well, they are just called low income workers in third world countries who make the technology we use in the west. Because if a company is earning billions in revenue, paying a worker $10 a day is cheaper than a slave master in roman times feeding them food ;)

And workers today only have the illusion of choice, since the economy is the master today.

People believed human slaves are not sentient?

Simpler. They didn't really believe them to be human in the same sense as themselves.

Then I don't think it's a similar reasoning to be honest.

One recognizes entity's rights based on it's similarity to the observer, the other recognizes them based on assumed consciousness level of the entity.

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.