Martin Panter:
> I think the basic idea of adding the warning is good.

Cool.

I pushed an enhanced version of my patch:

* I fixed _execute_child() to set correctly the returncode attribute
* I splitted the patch into multiple commits and I documented the doc
* I fixed test_subprocess on Windows


Me:
> TODO: fix also the Windows implementation of _execute_child().

subprocess.py on Windows is correct, but I had to fix more unit tests specific to Windows in test_subproces.py.


Martin Panter:
> One potential problem is how to provide for people who really want to let the child continue to run in the background or as a daemon without waiting for it, even if the parent exits. Perhaps a special method proc.detach() or whatever?

While I'm not convinced that the use case exists nor that it's safe to delegate the management of the subprocess to a different object after the Popen object is destroyed, I opened the issue #27068 to discuss this feature enhancement.