Arguably your phones notification feature is the unified feed. Theoretically it is possible (on Android) to write a custom home app to display those notifications without dismissing them.
There’s a launcher called T-UI I used to use when I had an android. One of the features it allowed was this. It would display a log of all your notification on your Home Screen and you could just click it and go straight into the conversation as if you’d clicked the actual notification. And u could always scroll up and view older notifications because they weren’t dismissed.
Sounds like a potential privacy & security risk, especially if the app can send messages too.
What is it gonna do, expose your Facebook data to untrustworthy corporations?
Yes, and that's why apps on Android need to explicitly ask for permission first. Giving the app that permission is a declaration of trust by the user. Not letting people do that would be a disaster for user agency in my opinion.
GDPR and similar laws also mandate clear user consent.