We should make sure we have EventLogging that logs when someone opens a cross-wiki notification group item, so we can see how many people use this feature. Once T121929: Clicking a NotificationGroupItem should expand it is fixed, I imagine we might already be sending notification-link-click events with an ID of -1, but it would be good to have a clear, separate action logged for this as well.
Also related: T120158: Fix EventLogging for cross-wiki notifications