No, they should not be removed. They describe differences between C++ standards, and so should be documented in the standard. Those differences don't disappear just because implementations backport them.

The behaviour of implementations should be documented by implementations, not in the standard.

I'd expect a brief note that describes the reality. Otherwise readers could be misled.

Users are not reading the annexes of the standard

These are informational notes about changes between revisions of ISO standards. If you want to know about real world implementation status, look elsewhere.

Agreed with @jwakely ; the standard is not the manual for your implementation. I think the phrasing in the Annex is clear that it discusses differences between releases of the standard.

Should we say which changes in Annex C are officially defect reports?

There are no defect reports for C++, since we release a new standard every three years (that, officially, is unrelated to any previous standard).

We would have Defect Reports if we would publish a TC (Techincal Corrigendum).