> Good email software lets one change the subject in a reply, but retain
> the in-reply-to header, whose value is a message ID.

Presumably most email clients get this right nowadays since on the
Scheme lists the threading always seems to be correct even when changing
the subject. That's reassuring: if the information is there, it's just a
matter of gradually improving user interfaces. Then even old threads
from archives would be displayed according to individual preferences.