They don't admit solitary confinement is torture right at the top (which is telling), but if you scroll down in the wiki you'll find:

Solitary confinement is considered to be a form of psychological torture with measurable long-term physiological effects when the period of confinement is longer than a few weeks or is continued indefinitely.

So who came up with this wonderful concept? In a plot twist, it turns out it was the polite, well-dressed oatmeal people all along:

Quakers and solitary confinement: We thought it was a good idea. Now we don’t! (CT Mirror 3/17/2021)

Oh well, we all make mistakes, right? The final reveal is that they may not at all be who you thought they were:

The SOCIETY of FRIENDS looks like another JEWISH FRONT (9-page PDF)

The usual caveats accompany the work of "Miles Mathis", but when you dig into the Quakers beyond what's included in the paper, it just looks worse and worse.