These headers can conflict with headers in the DKIM signature;
and parsing the DKIM-Signature header to determine whether or
not we can safely add a header would be more code and CPU
cycles.
Since IMAP seems fine without these headers (and JMAP will
likely be, too), there's likely no need to continue appending
these to every message. Nowadays, developers seem sufficiently
trained to use URLs with Message-IDs in them. So drop the
headers and save some cycles and bandwidth all around.