Undocumented features (e.g. argparse.REMAINDER)

I have a small helper utility that replays commands on subdirectories. Because it needs to be able to take arbitrary flags and commands, I found (I forget where, it was a while ago) about argparse.REMAINDER. (as an aside: I didn't want to use parse_known_args since it would break for common flags).

This was perfect for me but I can't find it anywhere in the API reference.

Is it common for there to be undocumented features? Does this count as one? Is it a risk to use this? Or am I just missing something?

Thanks!