What you have done is found something that is an internal ACF thing $field[‘class’]. Yes, you are correct, if you add something to this during prepare field it will be added to a basic fields class attribute. It may work with other fields as well, I don’t know.
It does not work on the date picker field because that value is not used. A date picker field is a jquery date picker and it’s made up of multiple fields and ACF applies different classes to each field that’s needed and the jquery date picker code takes if from there.
I think this is something that was not intended for public use, I would not use it in this manner because it is not consistently used.
As far as updating the code, you’d need to take that up with the developer https://www.advancedcustomfields.com/contact/. I am just another user trying to help out.
