While I think prolonged solitary confinement is genuinely harsh punishment, i'm ambivalent about banning it, because there really are prisoners who will take advantage of any weakness in communications control to wreak serious havoc in the outside world, and they can be incredibly ingenious about it unless totally locked down.

Also, as someone who has spent more than 24 hours in solitary confinement, I can say that doing it for a full day is pretty mild. You contemplate, use the toilet as needed, eat what's given to you, sleep and the time passes pretty quickly unless you're also being abused in some other way such as temperature extremes or having your clothes removed (I've had to deal with both and they make even brief solitary much worse)

Doing it for long stretches is a different story of course, but a day or two without "special" and basically abusive measures as mentioned above is very mild unless you have one very pathological aversion to any solitude. Claustrophobia could be a factor too but since prison is a confined place either way, it never fully stops applying (though Solitary cells are more claustrophobic than prison living outside of full lockdown.)