Yes, the structure for the packaging guide has changed a few times, but hopefully, <knock on wood>, we've reached the tail end of that. It's currently marked up with numerous FIXME comments, specifically on the "Advanced Topics" page:
https://python-packaging-user-guide.readthedocs.org/en/latest/additional.html
Beyond the guide, there's always the individual project docs: pip, virtualenv, setuptools, wheel, distlib
(btw, my plan is to refactor most of those to a consistent structure and style; I have issues open for the refactors in the user guide tracker; I'm currently working on pip and setuptools)
As for the
python.org docs, Nick Coghlan has already added notes to the top of the old "Installing/Distributing Python Modules" guides pointing to the new packaging user guide. Eventually, both of the old guides need to disappear in their current form, and be re-organized just as a distutils reference that covers install schemes, commands, options etc... and that's self-aware of it's place in the current ecosystem.
As for ensurepip/PEP453, I don't think that changes too much of the user guide documentation, except for a few places on installation, and to make "pyenv" more prominent (which is currently mentioned in footnotes only). PEP453 is already mentioned in a couple places in the Guide.
--Marcus