There is a multiprocessing spawn method on Python 2.7.  And we should never do such a crazy huge feature backport.

IMNSHO - We should not change the default in 3.7 either.  That is a notable behavior change.  That decision is entirely up to Ned (RM).  If people are using an OS that changes behavior out from underneath their application between minor OS version upgrades, they should take that up with the OS vendor.  We cannot workaround this fundamental problem.

Applications and libraries will still need to support a slew of Python versions and OS versions, so they're all going to need to be modified to explicitly request the "spawn" method from multiprocessing on macOS anyways.  So there seems little point in changing the default within a patch release 3.7.4.