> Modules are already cleared in the reverse order.

In Python 3.9, _PyImport_Cleanup() of Python/import.c contains the comment:

        /* Since dict is ordered in CPython 3.6+, modules are saved in
           importing order.  First clear modules imported later. */

But using Python 3.9, the z.py example output is:
---
a
b
c
---

The most recently imported module is the last one to be deleted.

The first loop iterates on sys.modules, not on reversed(sys.modules).