It's mandatory if you allow or use other 3rd party auth (ie. Facebook or Google).

Appreciate this - I can't say it makes sense because some third auth might be specifically selected for user-type or industry alignment (LinkedIn, Github, Microsoft login, etc).

M365 login specifically can hook into quite a bit of organization information along with the user itself that can help configure apps, wouldn't be relevant for Apple in those kinds of B2B scenarios.

Apple ID is almost always a personal or individual one.

It can be convenient for sure, makes me think about how there's other things where Apple is/was the only choice for the most part like payments, although that's changed for the time being.

It was our mistake to allow Apple Sign In. We wanted to make things better for users, but didn’t expect Apple to suddenly break their own login emails without warning.

Apple logins are useful for sure, I still use it sparingly, not ideal to have so many identity providers and they're all trying to vy for it.

Most people are where they started, whether its Microsoft, Gmail, Apple, etc.