This is the resolution to this thread:
The email mentions Python 4. Was this a joke or does anyone know more about that conversation?
There was originally a debate about whether the version number after 3.9 should increment to 4.0 or to 3.10. The decision now has been made in favor of calling it 3.10.
As far as backwards-incompatible changes are concerned, there is no requirement that the major version be incremented for that; Python did backwards-incompatible changes all through the 2.x series, and has done them in the 3.x series as well. There is a process for it which stretches out over a multi-year span to give people time to prepare.
Hats off to the Python team for being pragmatic and postponing this feature.