One thing I've noticed, as someone who's helped out on the macOS side of things for an open source project or two, is that the Mac/AppKit/Cocoa documentation issue is a barrier here.

There are absolutely OSS maintainers who wouldn't mind patching things for macOS... if they could figure out the expected behavior/etc. I've fixed things for projects that I only know about from plumbing around in AppKit for a few years now.