I know it is blasphemy to reply/reopen an old thread, but I think I have some sort of workaround for this. Say that per-category header warning message is 1) not going to be as native functionality, or 2) still waiting for plugin for that, I think a pinned thread atop all threads would suffice.
Netgate forum email notification (message-id)
is it possible to configure email forum notification to conserve the same message ID for the notification of a same topic.
I receive a lot of email and not being able to sort them by thread in my mailer is a problem (and worst on mobile phones mailer).
See here for original post.