It’s not very surprising to see implementations of non-accepted standards in the wild, since one usually needs to have at least a prototype implementation for a standard to be accepted (in order to avoid standardizing things that are found during implementation to be impractical or less ideal than expected). Furthermore, it can take time before a standard is accepted, which can lead to people implementing the PEP before it gets blessed as a standard.
This is not very different from how, e.g., C++ standards gain features that have already been implemented by major compilers as non-standard language extensions.
This specific part of the standard has indeed been a problem because of some misunderstandings during the PEP approval and implementation process. See the long thread Change in PyPI upload behavior. Intentional, accidental, pebkac? - #78 by barry .
I’m not aware of other known deviations from the wheel standard observed in practice.
Yeah, the spec says “The initially supported paths are taken from distutils.command.install.”, which should be updated now because distutils has been removed. I think the authoritative list is that of sysconfig schemes, can an expert confirm?