First a nitpick in the naming. If we’re moving this module, let’s rename it from MacOS X to macOS (or its lowercase equivalent). I’d be tempted to call the package and distribution platforms.macOS.libfile, although I recognize not a lot of the ecosystem utilizes namespaces and platforms is already semi-taken.
I’m happy to see this module incorporated into Setuptools, especially if it’s used privately. If it needs to be exposed for public consumption, it should be in a separate package. Setuptools would like to move away from being used as anything but a build backend.
As for ownership/maintenance, I welcome Anderson to own it, within PyPA or not. Personally, I’d rather see it within some organization than hosted by a single individual, and given that it’s already hosted by pypa, it seems a reasonable step to keep it hosted here, reducing the bus factor. I’d also be happy to host it under coherent-oss or see it added to jazzband if PyPA doesn’t want it.
My preference is for it to remain under PyPA whether in Setuptools or as a separate package.