I "watch" a few forums. Occasionally these watches are cancelled. This has happened at least 6 times over many years. Most recently, my subscription to the admin forum was cancelled maybe 3 weeks ago. This does not affect my access; I just don't get email notifications.
Is there a rule that cancels a forum watch automatically?
When BPP and E+ had separate forums, my watches on both of them were cancelled, causing me to overlook requests for assistance. These forums had low activity, and it is likely that the 90 day limit caused this.