Who "bent the knee" first? Itch or Steam? nope, Visa, MasterCard, PayPal, and Stripe bent-the-knee to "Collective Shout" first. Well to be honest, the payment processors have been trying to censor for awhile. Collective Shout I think was just throwing gasoline on a smoldering dumpster fire.
The reason why Itch (or Steam) cannot give a detailed list of what is and isn't acceptable, is that the processors have not said. There is a reason the Steam rule 15 is just this:
"15. Content that may violate the rules and standards set forth by Steam’s payment processors and related card networks and banks, or internet network providers. In particular, certain kinds of adult only content."
Key word "may violate", it is specifically not worded as "do violate". I have yet to see the payment processors specifically say what content they are censoring. The only public consensus is something related to "adult" content, whatever that means.