Apple doesn't allow adult content in their App Store, but they don't do anything to prevent adult sites from loading in Safari, even on iOS--never have. They seem to draw a distinction between their walled gardens and the open web. I'll admit I'm not an expert on Face ID and Touch ID for the web, but to me it looks like a feature of the Safari browser, not a walled garden.
On the contrary, although not against all uses within Safari, there are rules against use of Sign In with Apple depending on what you are doing, including one against pornographic websites as mentioned.
Implementing a client with this also works on Android and any computer that supports some kind of hardware authentication mechanism, like fingerprint or face recognition.
