While it is true you'll never find an app for PornHub in the App Store, there are plenty of popular apps filled with explicit, pornographic, or otherwise adult material all over the App Store.
The first native porn app for iPhone called "Hot Tub" may have passed notarization checks, but Apple wants everyone to know it certainly doesn't "approve" of such dangerous things on its devices.
Apple draws a clear line in the sand that it would never allow such an app on the App Store.
It seems the hill it wants to die on is that you can't go to the App Store and search "porn app" and get any results.
However, alternate app stores in the EU can publish anything that passes a notarization check, and that's happened with the first-ever native porn app for iPhone.
