It’s not even mandatory unless the app offers other auth providers like Google and Facebook. If your app is based around old fashioned email and password sign-in you don’t need to include Sign In with Apple at all.
But ugh please do. From the end user's perspective I 100% do not want to navigate your app's account creation process bouncing back and forth between my email client and your website and your app (if the redirect works, which it probably wont), and I 100% do not want to give your app my actual email address. Just implement apple's Auth flow. It's not that hard.
And the reason the redirect is broken, more often than not, is because there's analytics sitting somewhere in the stack that's either not working properly because it's inherently brittle or because it's getting blocked by my adblocker.
I understand the need for analytics in some situations but registration, signin, recovery, etc are critical paths and every layer of indirection is playing with fire.
Yeah, refusing to implement sign-in with Apple is anti-user: as a user I want to use sign-in with Apple (and Apple Pay) everywhere I can, forcing me to use some other product for these functions is not good UX.