Victor raises an important question:  should the *default* start behavior be made consistent across platforms?  Assuming we change it on MacOS, the default start behavior on Windows and MacOS will be spawn but the default start behavior on Linux and FreeBSD (among others) will be fork.

Reasons to consider such a breaking change:
* This inconsistency in default start behavior on different platforms (Windows versus not) has historically been a significant source of confusion for many, many users.
* These days, the majority of users are not already familiar with the rule "fork-before-creating-threads" and so are surprised and confused when they fork a process that already has spun up multiple threads and bad things happen.
* We are changing the default on one platform (MacOS), which should prompt us to consider how are defaults are set elsewhere.

Reasons to reject such a breaking change:
* Though changing the default does not break everyone's code everywhere, it will require changes to any code that depends upon the default start method AND depends upon data/functions/stuff from the parent to also be present in the forked child process.