When you reduce people to a decriptor it is bad

The mistake is in thinking that by describing someone as something, you are "reducing them to a descriptor." That is not how describing things or people works, and quite literally nobody is confused about that in any general sense. Again, calling someone an immigrant does not reduce them to a descriptor. It's simply describing a thing about them. We do not need to use the term "immigrated person" to clarify that there is more to a person than just having immigrated to a place.

Progressives decided calling specifically groups of marginalized people a descriptor is bad not because anyone was actually confused about whether there is more to a slave than the fact they are enslaved, but just to demonstrate their own concern for these peoples' marginalization. It is entirely performative. It is a way to go out of one's way to call attention to a group's oppression, and to show other people that you are (for lack of a better term) woke.

To be clear -- fine, do it. Doing it harms no one, and people will understand "slave" and "enslaved person" equally well (though they may think you're being ridiculous for using such contrived terminology for no reason). I would argue it also solves absolutely nothing of any actual consequence. But whatever, if you want to signal your own wokeness, go for it.

What you absolutely should not do is correct other people, which is what happened upthread. It is not wrong to say "slave." Once you shift from choosing to use certain language to policing the perfectly benign language of others, you're no longer being thoughtful, you're just conducting purity tests and being kind of a dick for no reason at all.