The point is that that's not your call to make. It's the project owner's call. That's what the build backend specification in pyproject.toml is for. If you're going to just ignore that, what's the point of having it at all?

> in the sdist (using information from pyproject.toml plus the developer's source tree to produce PKG-INFO)

Which is the only place that's relevant for this discussion, since we're talking about Gentoo, which builds everything from source.

> The latter cases mean that the developer is asserting that code needs to run on the end user's machine in order to get correct metadata. This is… usually questionable at best, and IMO impolite without a damned good reason.

I agree with all this, but as above, it's irrelevant to this discussion.