As identified by MDLSITE-3618, it can be confusing having the 'subscribe' links from emails simply toggle the subscription status because the state of their subscription might've changed since the email was sent.

An example scenario which happened to Fred:

1. The forum is initially set to 'Auto' subscription mode
2. Forum posts are sent to all enrolled users
3. The teacher identifies this was not desired and changes the subscription mode to optional
4. A student reads their email sometime later and clicks the 'unsubscribe link' to unsubscribe as they don't want to recieve the email

Expected result:
The student is unsubscribed from the the forum. (From a practical POV it could probably say 'you are not subscribed to this forum' or something), imporantly it shouldn't take any subscribing action.

Actual result:
The student is actually subscribed by this action because the mode has changed and it toggles the status.