A difficulty with writing a public API is that useful changes can occur at almost any level of the class.

The existing subclasses modify small, but deep methods, ones that handle things like line wrapping.

On the other hand, most of the issues you cite deal with higher level issues, either the top most method of the formatter, or alternatives to parser.format_help or parser._get_formatter (which create and populate a formatter object).

I've also responded to issues dealing with intermediate levels things, like the need for a more robust usage formatting, and changing how help lines are constructed.

The argparse documentation is a mix of how-to and formal API.  Mostly it describes how to perform common parsing tasks.  Even though argparse consists of classes, the documentation does not formally describe the classes, and their subclassing and all methods.  It's not a reference API.