Before, the Court began misapplying the Eighth, this couldn't have really been a question for the court unless some long neglected statute was used by an overly ambitious prosecutor.
But you have had some, e.g. Byron White, Warren Burger, and, if I’m not mistaken, Neil Gorsuch recently, who don’t quite interpret the Eighth as I think you suggest, but do apply a similar idea to capital crimes in the Fifth.
Now I wouldn’t be surprised if they, especially Burger/White, not originalists, found slavery to be cruel and unusual. Applying the standard 8A jurisprudence we have nowadays - i.e. evolving standards of decency to strike down legislatively authorised punishments, though deferential, do you think 13A would preclude this finding?
Why bother with an Amendment when you can use the original Construction itself?
No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.
All the Thirteenth does is limit how a Person might come to be held to Service or Labor.