FCM is integrated in kernel, so it cannot be killed. And it starts a persistent TCP connection to Google server. If all apps uses FCM, then all notification would route through this connection, and this significantly reduce battery consumption than the alternative, where each app uses its own web socket server.

So, assume using a degoogled android, can this notification replaced FCM if more apps use this notification server? This server uses web socket, so what does FCM use for its connection? Does web socket battery efficient enough?