IETFHDR demands strict compatibility with IETF standards for mail, which impose a very strict set of rules on LISTSERV:

LISTSERV MUST add a "Received:" field to the message as it passes through the server;

LISTSERV MUST add a "Message-ID:" field if it is missing;

LISTSERV MAY add a "Sender:" field if desired;

No other changes of any kind to the email headers are allowed.

Thus, if IETFHDR is specified, strict adherence to these rules prevents LISTSERV from appending subject-tags and/or making other modifications to the email headers.  Clearly this causes some head-scratching moments when it comes to implementing features like the list header fields recommended by RFC2369 ("List-Unsubscribe" and so forth), DKIM signatures, etc., not to mention simple things that people actually want, like subject-tags on their mailing list emails to make it easier to sort their mail.

For a long time, LISTSERV hewed to the standard and did not allow IETFHDR violations like subject-tags.  However, since LISTSERV 14.3, adding "Misc-Options= IETFHDR_SUBJECT_TAG" to a list header causes the IETFHDR option to always include subject tags for subscribers set to that header option.

Because, as already noted, this can be considered a violation of the practice for IETF-style headers, it can be prevented site-wide by the site administrator if desired, by setting the DEFAULT_MISC_OPTIONS site configuration variable as follows: