A lot of the "what's new in Python3" were written at past points like Python 3.5, it's now at Python 3.8. The latest release notes only cover the latest changes, but will miss everything before.
Certainly that will have too much information. u"" strings were in Python 2.x, removed in Python 3.0, and put back in a later version.
But there aren't many of those.
I want to know essentially "diff python2.7 python3.8". Yes it is the "cumulative What's new" -- I wanted to know if anyone has consolidated this.
For one, when you write "comprehensive" .. there's a lot of bug fixes too. And some of them might have come and gone too. Going through each one to give a pure 2.7-to-3.9 delta is mind-numbing.
You'll probably waste more time on HN today than reading through the things which were changed and then unchanged, or added in parallel to 2.x and 3.x.