Notifications on your phone are really powerful real-estate - they have the ability to: (1) encourage an app-open (2) inform consumers of something new (3) and at-best complete a full purchase.
So much so that (large) companies often have entire teams dedicated to deciding what notifications get sent / how often / to whom (ex: figuring out whether a friends bday notif > a video recommendation for Daksh on a Friday afternoon)
As such - I feel like today's notification ecosystem is quite messy. Especially since notifications are a (somewhat) COMPETITIVE mechanism for all apps to achieve goals 1/2/3. There's a limit to the notifications a consumer wants to see in a day + there's a marginal curve of sorts to the effectiveness of notifs from a single source.
On the consumer side, notifs also a 'regulated' form of communication that your OS handles (iOS / Android handle notifs that limit what you can & cannot do + lays out notification APIs).
The future of notifications = customizable, prioritized, and perhaps handled by 3rd party so that everyone can use their mini-computers (phones) to their max effectiveness.