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?