> Using close_fds=False, subprocess can use posix_spawn() which is safer and faster than fork+exec. For example, on Linux, the glibc implements it as a function using vfork which is faster than fork if the parent allocated a lot of memory. On macOS, posix_spawn() is even a syscall.

On Linux, unless you care specifically about users with Python 3.10+ on older kernels, implementing support for closerange() syscall in subprocess would provide better net benefit. This is because (a) performance advantage of posix_spawn() is no longer relevant on Linux after bpo-35823 and (b) supporting closerange() would benefit even those users who still need close_fds=True.