preexec_fn has been mentally and advisability deprecated for years. :) 
I'll mark it officially with pending deprecation in 3.9 destined to be removed in 3.11.  tracking that in its own rollup issue https://bugs.python.org/issue38435

As far as posix_spawn goes, I expect these kinds of between fork+exec features to be something that prevents posix_spawn from being usable.  As are many other things.  People who want to use posix_spawn will need to know that and seek to avoid those.  That's a documentation issue first and foremost.  Our existing POpen API is very old and wasn't designed to make that nice.

A new API could be made that *only* supports posix_spawn available features if you want an entrypoint that encourages the generally lower latency posix_spawn path.  (A subprocess.spawn function similar to subprocess.run perhaps?)  That should be taken up within its own enhancement issue.