It's not full chattel slavery such as was legal before the 13th Amendment, but the word "slavery" has always encompassed definitions short of that, e.g. in ancient Rome.
by which definition of slavery do we have “purchase and sale of humans” as part of that definition?!
Article 1(1) of the 1926 Slavery Convention: “Slavery is the status or condition of a person over whom any or all of the powers attaching to the right of ownership are exercised.”
just because you are not purchased/sold does not mean your condition cannot be defined as slavery
"Chattel slavery".
I don't believe there's a need to soften language to attempt to weaken the narrative of a "prison slavery system". If one is a proponent of forced labor for convicts then just say so: plenty of people will agree (and plenty will disagree).
The purchase and sale of humans, or the lack of such transactions is a meaningless distinction?