Maybe we should also rewrite the From header field so that people with dysfunctional MUAs won't have trouble replying to the list?
Maybe we should also rewrite the Reply-to field so that it doesn't matter when people get confused about the difference between reply to author and reply all?
And let's be sure to rewrite the To field so that everyone who uses that field to collate list traffic will get the messages put in the right bin.
</sarchasm>
It's taken us ~15 years to get rid of "features" like those mentioned above that well-meaning authors of list software put in to work around lack of user agent functionality. We're still not rid of all of it.
I used to call it "header munging disease" - the idea that if a message passes through an intermediary, there's a strong temptation for the author of that intermediary to consider munging every header field (as well as the message itself) just in case it could somehow be useful to somebody -- never mind that this removes valuable information from the message, and reduces everyone's ability to use the messages to that of the least capable MUA.
Putting [foo] in the subject header is just another example of this trend. Sure, it might be useful to people with dysfunctional MUAs, and there are a lot of those people out there. There were once a lot of people whose MUAs couldn't do "reply all", too.
The short-term solution is to make [foo] an optional feature specified on a per-recipient basis.
The long-term solution is to fix the MUAs to recognize and do appropriate things with List-* fields.
A tag in the subject line is clearly overdue. But, if we're going to do
it, let's do it right. Please use "[IETF]" not "[ietf]" because it's
more befitting of a proper acronym. If people would really rather
defile the IETF's good name by calling it the "ietf" then maybe we could
extend the mailing list software to allow each user to define their own
"turd" to place in the subject line of mail they get from the IETF list.