Push notifications work on GrapheneOS whether apps do it themselves, use UnifiedPush with the user's choice of provider or use FCM. UnifiedPush and FCM are a more efficient design where apps share a push connection. Unfortunately, many apps only support FCM and some support their own push as a fallback, but few support UnifiedPush. FCM works very well via sandboxed Google Play, which is an approach where Google apps can be installed as regular sandboxed apps with zero special access or privileges. Nothing FCM does actually requires special privileges and our compatibility layer makes it work without it.

GrapheneOS does not include sandboxed Google Play but rather includes an open source compatibility layer providing support for installing Google Play as regular sandboxed apps. They can't do or access anything more than other apps including the Google Play code running inside apps using Google Play which is the reason for choosing this design. It simply uses the same app sandbox and permission model which are both greatly improved by GrapheneOS for supporting running the rest of Google Play not bundled with apps using it.

Worth noting apps don't need Google Play services to use Google services and many Google libraries like Ads and Analytics work without it. FCM requires Google Play services but many of their libraries do. There are Lite variants of Ads and Analytics for keeping apps smaller which lose the ability work without Google Play services. The general reason for the design is they don't want to have huge apps and want to be able to update the clients for their services without app developers doing it and shipping an app update. FCM is one of the special cases requiring the central design for efficiency. UnifiedPush is an alternative with choice of implementation / provider.