> I agree that this is an improvement, but isn't it a bit late for removing a public field from a public header file in 3.4, without any preceding deprecation?

The header is not public, it is private. The structure in not
documented (in Doc/c-api/*.rst). There is only one public documented
method: PyFrame_GetLineNumber().

If you rely on such implement detail (PyFrameObject field), you have
to be prepared for breakage between Python releases (3.x).

Do you know applications relying on the field?

How don't see how you would like to deprecate the field since it's
private and not documented.