I couldn't find any documentation of how to implement push notifications in maui. Maybe somebody was able to integrate xamarin library in maui project to accomplish it?
.NET MAUI is still in preview so while "kicking tires" is happening a lot, no one should actually use this in production. Bits are still in progress, as well as documentation and third-party libraries.
Having that said, concepts like push notifications actually won't change that much. Push notifications still happen very much on the platform-specific layer. If you, for instance, follow this guide, you'll see that it speaks of a MainActivity for Android, that will still be there, although in a different place. Same for the AppDelegate in iOS.
You have to translate some concepts to .NET MAUI but it shouldn't be too complicated to figure it out. But if you can't and have time, wait a bit longer the libraries and documentation will catch up!