Interesting! I do think that there’s packages that do both – indicate their license as full text as well as just-the-name.
There are several packages where the license field include the text of the whole license instead of just a few letters indicating the given license.
FWIW, solving the “Python package licenses are a mess” with SPDX-based license declarations is out of scope for this PEP – that’s what “PEP 639: Improving license clarity with better package metadata - #74 by pradyunsg” is about. That said, as noted in the PEP (and reiterated by @brettcannon here), that is something that we’ve accommodated for – by keeping the string value of the license attribute “open”, so that it mapped to the SPDX-based license metadata field, if/when that gets formalized.