Documentation of the issue for sure!

A RuntimeWarning is a maybe if it can be made conditional on the OS version (10.13 and higher?)... when the "fork" method would be used.  BUT be very cautious about warnings as they tend to crop up in front of users rather than developers when it isn't their fault.  So if there is any flaw in the logic of when to show it in a situation where the application isn't about to crash or lockup, it'll show up when it shouldn't and cause somebody some grief to deal with via new code to ignore that warning.