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Unless it could replace the fork+exec code path in its entirety, which I do not believe is possible, I see posix_spawn() as a distraction and additional maintenance burden with no benefit.
http://pubs.opengroup.org/onlinepubs/7999959899/functions/posix_spawn.html
Read the RATIONALE section. The posix_spawn API was not created to make subprocess creation easier (i'd argue that it is the same burden to setup a proper call to posix_spawn as it is to do everything right for fork and exec).
One notable thing posix_spawn() does not support: setsid() (start_new_session=True) of the child process. Obviously it also couldn't handle the arbitrary preexec_fn but preexec_fn is in general considered harmful.