I kind of disagree. For me and few smaller forums it is almost every day issue. I would say when a forum is… very content centric and first post is more than a question and longer there is and will be situations when something important has added or changed on OP. Kind of same thing than with wikis but by one user only.
I normally write one liner and tell what is changed or added and will delete that post after a while or after next post in that topic.
How that should notified to users… I don’t know.