For derivative Python implementation there is a standard strategy: they uses a copy of CPython standard library.

Tthat's why any new module should have a pure Python implementation and that's why https://www.python.org/dev/peps/pep-0399/ exists.

Jython has copy() method as well. IronPython has it. I doubt if an implementation without xml.etree.ElementTree.Element.copy exists at all.

The method removal breaks backward compatibility without a reason, __copy__ alias is valuable and backward compatible change.