From the StopIteration-in-generators handling transition plan (https://www.python.org/dev/peps/pep-0479/#transition-plan):

* Python 3.5: Enable new semantics under __future__ import; silent deprecation warning if StopIteration bubbles out of a generator not under __future__ import. 

The first half of that was implemented in issue 22906, but we missed the second half.

The reason the latter is needed is so that people can turn the DeprecationWarning into an error to test for compatibility with the future default behaviour without needing to add the new future flag to their code (which would break compatibility with older Python versions). Ram Rachum pointed out the absence on python-ideas: https://mail.python.org/pipermail/python-ideas/2015-May/033580.html