subprocess's preexec_fn feature needs to enter PendingDeprecationWarning state in 3.9, to become a DeprecationWarning in 3.10, with a goal of removing it in 3.11.

Rationale: We now live in a world full of threads, it is entirely unsafe to call back into the python interpreter within the forked child before exec per POSIX specification.

We've also already made preexec_fn no longer supported from CPython subinterpreters in 3.8.

If there are not already issues open for desired features of subprocess that do not yet have replacements or workarounds for *specific* actions that preexec_fn is being used for in your application, please open feature requests for those.  (ex: calling umask is https://bugs.python.org/issue38417, and group, uid, gid setting has already landed in 3.9)