I don't think I'm adventurous as I try to use its public API. If something is public in the pythonic terms (no underscore before it), but undocumented, that can obviously mean two things: either the documentation is outdated or the API published something it shouldn't.

As I wrote in issue 46058 - which is the same issue as this because of the implementation in argparse.py - I tried to create a group hierarchy in a way that I can pass the responsibility of argument validation to the argument parser. I don't think the good practice is to reimplement the same behavior again and again in dozens of Python programs if it is something that could be handled in an argument parser. My original intent was to provide patch or patches fixing the issue but with the deprecation in issue 22047 this became pointless.

There were a few other opened issues indicating I'm not alone with this expectation.