This issue was brought up and closed as part of issue # 41592.

The add_subparsers method returns a protected _SubParsersAction instance. To add a sub-parser, the public add_parser method of the protected _SubParsersAction instance must be used.

1. It is unexpected to require the use of a public method of a protected class instance.
2. The protected return type breaks type hinting because
   1. protected and should not be imported
   2. it is not available for import

Real world use case example:

sub_parsers_action = parser.add_subparsers(...)

def build_subcmd(sub_parsers_action: _SubParsersAction):
    ...

The ask is to make the type of whatever is returned from add_subparsers public.