This problem also occurred this week on my iPhone 11 running iOS 15.4. Settings > General > Transfer or Reset iPhone > Reset > Reset all Settings disabled all the notifications from my 3rd party apps. The apps were not listed in the Settings > Notifications menu and the Notifications option was absent under the "ALLOW app_name TO ACCESS" heading under Settings > app_name so I couldn't get to the Allow Notifications slider.
The most time-consuming and enraging process was reinstalling and reinitiating my multiple vital authenticators and email apps.
Grrr!
Apple - Fix this!
BeachChimp
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