I'm splitting off part of MDL-35027 into a separate improvement.

The subscriber list of a forum may contain users that can't actually access the forum. For example, the forum may be set to auto-subscription but may currently be hidden ("closed-eye button"). In this case, all/most students in the course will appear on the subscriber list as seen by the teachers (mod/forum/subscribers.php), but they won't actually receive mailings from the forum since they can't see hidden activities. Alternatively, the forum may be hidden to some users (permanently or temporarily) via availability conditions.

It is in principle useful or the teacher to see these entries, since the teacher may want to edit the subscriber list before the forum is made visible.

However, the teacher should be able to tell from the subscriber list which subscriptions are actually active at the moment, i.e., which subscribers can see the forum. For example, "inactive subscription" could be displayed as grayed out.