The PEP you've linked to talks about eggs in relation to PyPI, but why are they no longer being created by Pillow?

I'm not sure. #4981 says the egg was created from the python3 setup.py install command. That command was changed to python3 -m pip install . in #5896, so it could have been that or a change in setuptools.

The line was originally added in #4992, to resolve #4981.

Yep, yhat was to prevent us uploading an egg when doing a manual release.

Now our artifacts are all generated and released from CI, so it wouldn't even matter if an egg ended up in a local dist/ when running make release-test.