I believe we must change the default behavior on MacOS to use spawn instead of fork.  Encouraging people to use fork by default on MacOS is encouraging them to create something that effectively will not work.  Keeping fork as the default behavior when we have already turned off all of the tests of fork behavior on MacOS also makes no sense.  Existing Python code that depends upon the default behavior (fork) on MacOS has already been broken -- if we make this change, we are arguably not breaking anyone's working code.

Users can and will still be able to specify the start mechanism on MacOS, including fork.  This empowers users to continue to handle even the most esoteric use cases without loss of functionality from multiprocessing.  Though admittedly, without an ability to test the behavior of fork, this will need to be marked as deprecated.

I will supply a patch making this change and updating the docs shortly after PyCon.