I’m seeing the pattern now. Even in forums more than a decade old (and before @ mentions became a feature on many sites), people used @username even if it did nothing. This was simply shorthand for getting a specific person’s attention in a thread with many participants.
Looks as though BuddyPress is trying to be smart and pull these @ mentions from old content and add them to the activity stream. Since it’s a new addition to the activity stream, BuddyPress also thinks it’s new and therefore sends out an email notification. This is why it only affects old members.
Now… How do stop it?
Stopping notifications across the board is not a good idea. Even if I do it now and turn it back on later, we’ll never know when an old member might join us again. So really the only solution is to have BuddyPress stop importing old forum posting with the ‘@’ character into a new activity.