> Changing the major version number itself is a breaking change

So is the proposed change, in a way. At some point, there will be a 4.0 release, which may or may not break the code in question. So, testing for "version_info[0] == 3" is already wrong, because that code almost certainly wants to know if it runs in Py2 and not if it runs in Py3. It's better fixed soon, before we get too close to Py4, and I think a deprecation notice could be helpful for that. Preferably for as many of the non-future-proof spellings as possible.