unfortunately there are very few apps so far that allow signing in with apple even though i see them offer email, google and facebook options. i think out of all of the apps that i use only adobe lightroom allowed me to sign up with apple.
https://developer.okta.com/blog/2019/06/04/what-the-heck-is-...
Why does a standard matter for justifying mandatory inclusion of ... anything? Most of their mandates probably lack any kind of standardization.
[0] https://bitbucket.org/openid/connect/src/default/How-Sign-in...
[1] https://appleid.apple.com/.well-known/openid-configuration
That seems like an abuse of monopoly power.
Shameless plug, I work for Skyscanner and we've had it since iOS 13 launch.