I wonder if this will lead to apps not allowing the initial sign-up inside the app, thus potentially reducing usage of AppleID? Although it wouldn't surprise me if Apple were to forbid apps that don't allow in-app sign-up.
A part from special cases (like bank apps), you have to always allow users to sign up in app. And if you allow to do that through identification providers other than email (eg. Google or FB), you have to provide Apple sign-in too.
Didn't Apple require that apps must allow for at least some functionality without external signup? I seem to recall that that was an issue for the "Hey" email client.
