LOL… You are aware of course that Slavery was NEVER made illegal in the USA. Well, not totally anyway. It was just “Modified.” Have you read the 13th amendment?
Until the adoption of the 13th Amendment in 1865, the Constitution did not prohibit slavery. (To Prohibit: formally forbid a person or group from doing something. There is some room for flexibility here.)
THE 13 AMENDMENT TO THE CONSTITUTION:
Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
The argument can and has been made; “Our prison system is institutionalized slavery”.
https://innocenceproject.org/13th-amendment-slavery-prison-labor-angola-louisiana/