Technically this would be a very complicated thing to do, permissions get tricky. What if user Jane can not see any messages with the tag #this-is-between-bill-and-bob
- Do we let Jane know the tag #this-is-between-bill-and-bob exists?
- Do we let Jane know that there are 17 messages she can not see with the tag #this-is-between-bill-and-bob?
Clearly the answer to both is no, but to make it technically work it becomes massively expensive to get the list of tags and counts.