Prisoners are defacto slaves.
HB1 are defacto slaves.
Outsourcing is defacto slavery.
Working for the same wage everywhere,that barely affords you food and roof with no chance to improve your fate, is slavery. Slavery is all around you.
Its just that the word is not used.
Perhaps "de facto slavery" was a poor word choice. That said, slavery as we traditionally understand it means you are entirely dehumanized; in essence, you are property.
The examples you provide certainly are challenging, to put it lightly, but they are not slavery itself. Though the closest, prisoners still have human rights. HB1s have human rights, and they also chose to live and work the US of their own free will. Outsourcing exploits access to cheap labor in a global market, which will serve to eliminate extreme poverty by providing work opportunities, though we also know that such opportunities have come at the expense of human rights by our Western standards, which, to my understanding, is typically caused by poor top-level oversight of local management. The issue of wages is a largely Western phenomenon where, for better or for worse, people are unwilling to make significant lifestyle changes (e.g., moving somewhere new) for a better wage, a wage that hundreds of millions of people around the world would love to have.
The word "slavery" isn't used in those cases because it's a false equivalence. Yes, there are countless ways in which humans attempt to control each other, but bonafide slavery isn't it.
To return to the point, the status of legal personhood provides certain rights. What if it's legal for a clump of Homo sapiens cells to grow into something that never meets the threshold for attainment of legal personhood? I believe that clump of cells may, in certain places, be considered property, which is truly chattel slavery.