I know from talking to a few course authors this past fall that a very popular request is to receive a notification any time there is a new discussion post. (cc: @john_curricume)
I also feel like as a user, I think I’d only expect to see notifications about the course I’m looking at and/or the course(s) where I’m listed as staff if this notifications sidebar were in Studio, but this is a guess. Would require more user research to confirm.