Beware, there's a difference between "push notifications" (which is what your links are talking about) and "notifications", specifically with the "notification history" feature.

Push notifications are a mechanism to transport messages over google services. What that does is that the backend service of some app (e.g. the Signal server) can send a message to an app that's currently not actively running to tell it that there's something new happening, e.g. a new incoming message. This goes via Google services because that way, the app doesn't need to be constantly running. Google services then wakes up the app and allow it to do something with that info, e.g. display a notification.

The alternative is that the app is constantly running, constantly actively checking for new messages and thus constantly consuming power.

This can be e2e encrypted by the app, and then Google can only see metadata.

Notifications, on the other hand are the things that show up on your phone when you swipe down from the top navigation bar. These notifications can be read in plain text by any app on your phone, including the OS. If you have Notification History enabled, they can be backed up (again in plain text) to Google's servers. And any old app you have on your phone can silently do the same. That's why Signal allows you to hide the text content and/or sender name for notifications.