Digging into the source code of the discussion it seems that notification is relevant for the first response only of a thread. ref.
That being said, I don’t think there is a feature or a flag that once enabled would allow the instructor or course author to receive an email.
It’s worth noting that discussion service future is unknown, hence:
That being said, it’s not clear what the path forward for this app is. Forum functionality is something that has not been actively worked on for a while, and it’s been suggested that we should remove this app altogether in favor of having better integration with other more established forum software. This decision is usually complicated by the desire to have tight integration with courseware and access to data for analytics.