As far as I can tell this was already working, even in 3.4.0a1 (the first pre-release of 3.4):

>>> tree.write('/dev/stdout', xml_declaration=True)
<?xml version='1.0' encoding='us-ascii'?>
<rss version="2.0" />

But it is not working in Python 2:

>>> tree.write('/dev/stdout', xml_declaration=True)
<rss version="2.0" />

Looking at the history, I am confused because a 3.2.3 revision (df8609f1854d) comes up in the history of 2.7’s /Doc/library/xml.etree.elementtree.rst. Perhaps it is just that the documentation has inherited some wording from Python 3. Possibly also relevant: revision 57e631f <https://bitbucket.org/mirror/cpython/commits/57e631f> and Issue 8047.