distutils.sysconfig doesn't expose the headers path either, it's only there as a default value for the install command (Lib/distutils/command/install.py).

So it doesn't seem unreasonable to provide a recommendation on where to put shared header files and let installers do their own calculation. If an installer wants to install into _another_ environment, it can't rely on sysconfig anyway, so we need the spec as well as any implementation. And if we don't have the spec then people will have to reverse-engineer the implementation anyway, so we may as well start with the spec.

Now, whether we actually need or want packages dumping all their headers in one directory is a different question. At least on some platforms they'll also need to import libraries too, and tools like Cython have different files yet again. Many existing project keep these files inside their package and offer the path on request (e.g. pybind11), so perhaps we actually want to standardise pkg-config-like metadata instead?

(Also crossposting back to the discuss thread)