Earlier quoted context omitted.
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I agree with you and don't really get what Apple gets from removing a valid Czech character, but how would you test if all existing passcodes remain inputable without knowing the passcodes of all iPhone users? The one way to do this that I could see is to include both the new keyboard and the old one and if someone fails to unlock with the new one auto report that to Apple (not the code, just that the unlock failed a…
Then you wait. Then you roll out a version where the new functionality is flipped on by default, but where you still allow to explicitly toggle to the old one. Then you wait some more.
And then - only then - you roll out a release where the old functionality has been removed entirely.