On May 28, 2019, at 17:38, Ned Deily <report@bugs.python.org> wrote:
> 
> Ned Deily <nad@python.org> added the comment:
> 
>> To be clear, what is unsafe on macOS (as of 10.13, but even more so on 10.14) is calling into the Objective-C runtime between fork and exec.
> 
> No, it has *always* been unsafe. What's new as of 10.13/14 is that macOS tries much harder at runtime to detect such cases and more predictably cause an error rather than letter than let the process run on and possibly fail nondeterministically.

Right, thanks for the additional nuance.  I think what changed is that in 10.13, Apple added a warning output when this condition occurred, and in 10.14 they actually abort the subprocess.