[nirava]

There is a list of valid characters accepted for a passcode. That list was created, the characters debated, and a consensus reached by Apple engineers (I hope, for all our sakes. I don't want to imagine a world where this bare minimum level of engineering diligence wasn't done by a trillion dollar company)

Just have an automated keyboard test for every new release to ensure those characters aren't broken.

[Matl]

Agreed, but just to be clear; I was asking how would you test that assuming you still wanted to remove a character that was previously present.

[nirava]

That's the thing: you don't! The charset for passwords should be always inputable even if no one is using it.

If you wanted to reduce the size of the charset, you'd basically create a transition plan, and ask everyone in the world with a passcode to set a new passcode and validate that against the new charset/rules. A company that can perfectly transition the world from x86 to ARM can surely manage that.