I updated PR 18738 to document the incompatible change in What's New In Python 3.9. Sadly, I expect that almost no third-party extension module implement the PEP 489 yet. So I expect that little or no third-party code is impacted in pratice.


> the module slots are then only needed if the module state actually gets populated, so we can safely skip them if it never even gets allocated

That's also my understanding.

> custom module subclasses should clean up like any other class instance

That sounds like a reasonable compromise to me.