4) Deprecate the function to be removed in 3.4 or 3.5 and "fix" it to always return [].

This way we won't break any 3.0-3.2 code that is using the function, but the users of such code will start to get DeprecationWarnings in 3.3.

There's no meaningful way to fix the function correctly, as the original header data isn't stored anywhere in HTTPMessage or its base class email.message.Message. The function is also obsolete: the get_all() method of email.message.Message can be used.

@stachjankowski: How did you find this issue? Are you porting from 2.x to 3.x or have new 3.x code that uses this function?