This can be done to make distro packagers happy, without introducing more issues around having extra build time dependencies or different versions of dependencies. https://mesonbuild.com/Subprojects.html#toggling-between-system-libraries-and-embedded-sources explains most of how that works. The one thing it does not show is how to restrict dependency(...) to a single version, but that is possible. That way, if the system has the right version of a library available already, it can be used. And if it's not found, we use our vendored copy (in the SciPy source tree or in a git submodule).

This came up several times:

• recently for Qhull, which conda-forge builds separately (see this mailing list thread),
• for Boost in BUG: problem with building scipy against system boost #15260 (Boost is less interesting because it's header-only, but still some people seem to care)
• the Fortran-lang folks have asked about relying on their Minpack in the future (could be vendored still of course)
• there's other libraries that I can't remember questions about, but may be present in distros (ARPACK-ng, HiGHS, etc.)

This is not high-prio, but it would be nice to implement.