Tab: Notifications > Enable Notifications. Options: Yes / No (default No). Global only.

This is the master switch for the in-admin notification feed (bell icon). When No, no notifications are recorded and the live notification stream reports itself as disabled. When Yes, events of the enabled types (see Enabled Notification Types) are stored and pushed live to logged-in admin users; the sound, popup and counter settings below then decide how they are surfaced.