Idea Summary
With the jQuery Date Picker you could add custom attributes to the input field of the date picker. E.g. to automatically format a date while typing we use onkeyup:

The new APEX Date Picker puts Custom Attributes on <a-date-picker>, which seems rather useless (all(?) attributes can already be controlled in APEX Builder):

The idea is to move Custom Attributes to the input field of the Date Picker.
Use Case
Backwards compatibility with the jQuery Date Picker.
Consistency with other input items where Custom Attributes are added to the input field.
Note: workaround for now is to run:
$("#P63_SOME_DATE_input").attr("onkeyup","formatDate(this)");
Preferred Solution (Optional)
Custom Attributes should be added to the input field of the Date Picker.