It should be great to display a message with a link on old releases pointing to new releases.

I can imagine a banner like:

This document is about an unsupported version of Python. Please check the doc about Python 3.7.

With the "Python 3.7" text being a link to the right page on /3/, so both user and search engines can follow it.

Already been discussed here: https://bugs.python.org/issue35435