The fact that several packaging PEPs have gone un-implemented in setuptools, and thus are effectively unused in the greater Python packaging ecosystem, because setuptools is really hard to work with since it’s everywhere.
Again, the backend is not the relevant place for this to be “stricter”. It’s not even a question of strictness. All the interesting configuration for this depends on the user environment and preferences and not on the project itself. The same person may develop the same project on platforms with and without good support for symlinks. Having a per-project configuration for such a thing would be annoying at best and useless at worst.
If you want to make the case that this is as good as we can get implemented in a reasonable timeframe, that’s fine, but please take the time to understand and engage with my actual substantive points of disagreement.
You and I sat down together at PyCon 2019 and wrote a PEP based on the “virtual wheel” version, not anything even close to PEP 660. Here is a description of the high level concepts, clearly incompatible with what is being proposed here.