2 points
You can simply log this response. Then developers know to replace that call in the next version. If you have unit tests that look for this then you can be sure that your next version is not using any deprecated API calls.
3 points
We don’t have many unit tests that test against live APIs, most use mock APIs for testing.
The only use for this header would be if somebody sees it during development, at which point it would already be in the documentation or if you explicitly add a feature to look if the header is present. Which I don’t see happening any time soon since we get mailed about deprecations as well.
