It's actually supported by mutt, dovecot[1], and likely some other
software to augment the Status: header. While dovecot doesn't
expose X-Status to clients, mutt will write 'A' (answered) and
'F' to X-Status (but not T (draft)).
So we'll drop it like we do Status since it's not suitable for
public mail, but stick it in an @UNWANTED_HEADERS array will
allow us to configure an override if needed.