As a broad methodology it isn't torture per say, sure. But the implementation in the vast majority of our justice systems 100% no question is aggregious torture.

Digestible source: last week tonight: https://youtu.be/_uSZwErdH3I?si=DfHkVDbGegVt06tJ

More detailed source: https://www.newyorker.com/magazine/2009/03/30/hellhole

It would be interesting to see you or anyone with your opinion about solitary spend even six months in U.S. Federal/state supermax levels of isolation and then emerge to say with a straight face and sound mind that it isn't torture.

Humans are innately social animals, and isolating them against their will from virtually all human contact even for fairly short periods of time can be more destructive to their being than physically mutilating them, even if the scars won't be so visible. Whether such a punishment is justified for some types of extreme criminals or not is another argument that we could debate, but let's at least be honest about describing it for the cruelty it is as a method.

Total supermax-type solitary is one very major reason why even some of the wold's hardest criminals in other countries fight extradition to the U.S at all costs. Should someone like Assange be placed at the same moral level as a mass murdering Mexican narco boss?

You have no understanding of prisons.