"In November 2002, a detainee who had been held partially nude and chained to the floor died, apparently from hypothermia."
https://www.thedailybeast.com/the-most-gruesome-moments-in-t...
(I think a better standard for categorically ruling out a tactic/punishment/procedure is "would I want this on my worst enemy/Adolf Hitler?")
"A society is judged by how it treats its weakest", "would I want $GOOD_PERSON to fall victim to this" -- those sound nice, but don't translate into a practical metric for appropriately navigating real moral dilemmas.
[1] I know the lectures about the evils of mass incarceration; those don't prove that no one should be imprisoned.
This may permit consideration of the relative social value of the punishment, alloyed with the emotional bias provided by the context.
This is about the worst standard I can imagine.
The standard was "If I wouldn't be willing to use it, even against Hitler, it should be categorically disallowed."[1]
That is, if you consider it so brutal that you'd feel queasy about using it even against someone really bad, i.e. a case where you would most favor casting morals aside, then you know it's beyond the pale.
It may not make sense if you've never experienced something so bad that you said, "wow, I wouldn't wish this on my worst enemy", but I'm sure at least some fraction of the population has had that thought.
[1] If necessary, replace "categorically disallowed" with "off the table".