Quoting Timo Sirainen tss@iki.fi:
Do you think I'd break a lot of people's filters if I removed the prefix? :) Anyone strongly for/against removing it? It seems kind of annoying to me whenever I happen to think about it.
List-Id has been mentioned as the replacement mechanism by some, but
the main issue is that it is not immediately viewable (at least with
any rationally configured MUA) to the user. Obviously, filtering by
List-Id is the preferred method, since it has the canonical mailing
list definition. However, List-Id filtering does not work in all
situations.
For example, a common situation (at least for me) is someone who
replies directly to your message from a list instead of to the list
address. This will most likely cause that message to end up in your
INBOX rather than being filtered into the appropriate mailing list
mailbox. Having the list name in the Subject can be useful to
visually filter these incoming messages in your INBOX, rather than
potentially deleting/marking as spam since often times you may not
recognize the sender.
FWIW, use of brackets in this manner is sort of a pseudo-standard,
insomuch as it is an acceptable component of Subject lines with
respect to threading/sorting pursuant to RFC 5256.
michael