While investigating the details of Python packaging metadata for a Setuptools issue, I learned that PEP 566 allows the Description to be indicated as a field or as the payload of the metadata and in fact the latter approach is what is seen in dist-info metadata as produced by today's tools.

That means that the PackageMetadata Protocol does not expose that information. For example:

draft $ pip-run -q setuptools
>>> import importlib.metadata
>>> dist = importlib.metadata.distribution('setuptools')
>>> dist.metadata['Description'] or 'missing'
'missing'

But the content is there:

>>> dist.metadata.get_payload()[:10]
'.. image::'

Probably importlib.metadata could copy that payload into the Description field, but it probably needs to first encode the content according to this spec (yuck).