ntfy 
ntfy is a versatile tool for notification about various events. It also supports UnifiedPush, an open protocol for real-time notifications.
To use ntfy as a central app for notifications, the app must first be installed on the phone. Then, apps that support UnifiedPush, such as Element, Fluffychat or Tusky can use ntfy as a push provider. This way, not all apps need to keep a connection open to the server and are more energy efficient as a result. In order for you to use my server, you must first set it up in the settings.
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My Offer
Since May 2023 I provide ntfy for free: https://ntfy.sp-codes.de You are welcome to use ntfy as push provider for your apps if you like.