I think there’s broad consensus that slavery only applies to human labor. Even within that spectrum people avoid the term (see forced prison labor). We also don’t use it for animal labor, for instance.
Human slaves were often considered to be less than human or, at the very least, not deserving of basic rights that other humans enjoyed, as part of the moral and ethical frameworks that supported the practice. I think we might see the same shift in dominant ideology if we do have “true” AGI. I’m sure I could be convinced that an intelligence that develops and grows over a number of years begins to have a right to exist and a right to freedom of expression and movement.
Given the outcry/backlash over Dall-E/ChatGPT (what is "real art", etc.) and how much of our society is permeated by a search for authenticity (perceived) already, I wonder if you're right. We might decide "artifical" lifeforms are a lower class than "evolved in nature". For many religions this could be a natural take - made by God vs. folly of man, etc.
The context uses human-like/human-level a lot, but I agree what level and type of intelligence commands human respect is tricky business.
> forced prison labor
Would be interesting if we found ways to convince ourselves the AIs had it coming.
Generally speaking, slavery has been morally acceptable and popular before, and I will also not be surprised if we return to those ways.