> But within Python 3, I think the deprecation system is decent. As far as I know, it really only occurs with "uncommon" APIs, so the vast majority of code will continue to work as expected on later Python 3 versions.

The problem is that every new Python version seems to break something. Python 3.8 was released almost two weeks ago, and some popular libraries still don't work with it, e. g. SciPy, Matplotlib, OpenCV, PyQt, PySide2 and probably a bunch of other libraries that depend on the above.