So, how to move this further?

In #13425 Petri proposes 4 alternatives, copying them here:

1) Document the function to make it officially part of the public API
2) Rename and move the function to http.server
3) Leave it undocumented and just fix it
4) Deprecate the function to be removed in 3.4 or 3.5 and "fix" it to always return [].

I assume 4) meant:
4) Deprecate the function to be removed in 3.4 or 3.5 and fix to do what its docstring specifies.

My proposal is a more explicitly spelled out version 2):
5) Remove the function, replace its usage in http.server.CGIHTTPRequestHandler and add a test for http.server.CGIHTTPRequestHandler that exercises the part that currently uses getallmatchingheaders since that's obviously broken now.

The rationale for removal without deprecation is:
* the function duplicates get_all so there's no reason to have it
* it's probably not used by any (working) code because it just doesn't work

Mike can you tell us how you found out about this breakage? Were you using the function? Did you use something else to workaround it since it's broken now?

Senthil, Petri do you agree with option 5)? If so I can provide a patch.