You can check with `curl -v`. tl;dr: the server doesn't support that header, but since the response does include a Last-Modified header, curl helpfully aborts

You can check with `curl -v`. tl;dr: the server doesn't support that header, but since the response does include a Last-Modified header, curl helpfully aborts the transfer if the Last-Modified date is the same as the mtime of the previously downloaded file.