There will also be cases where the sdist simply doesn’t know all the metadata for the final wheel, because it varies depending on what happens during the build. So we could think of this proposed field as “I can promise that my wheel metadata is not dynamic and will match the sdist”, rather than just “I don’t have a bug”.
Also if we did this, I think the trick would have to be that if you set this flag, then pip and other build tools need to actually enforce it, by comparing the sdist and wheel metadata and erroring out if they don’t match. That’s the only way to make it actually trustworthy.
But setuptools will never be able to set this flag automatically, because setuptools has no idea whether any given setup.py has tricky dynamicity in it. Which means that this flag would have to be something that individual projects have to opt-in to. Which is fine for projects that have active and diligent maintainers. … But those projects mostly distribute wheels already, so this flag is unnecessary. The projects that need it are the ones that only distribute sdists. Some of those projects do have active maintainers that could potentially be convinced to add this flag. But I think to make a real dent in the missing-metadata problem, you’ll need to find something that works for the inactive-but-still-used projects, and an opt-in flag won’t help with those.