Push messages are usually realized via Google's Firebase Cloud Messaging – which is proprietary and requires Google Services (or microG) installed on your device, requiring a persistent connection to Google servers. Privacy proponents rather avoid that. Without Push notifications, applications need to revert to polling if they want updates. While this is perfectly fine with asynchronous communication like Mail, it's very inconvenient with synchronous communications like chat and even more incoming IP calls in messengers.
Luckily, there are alternatives – finally! Now if only all apps would support them! Or at least one of them: UnifiedPush tries to combine them all (Gotify, NoProvider2Push and even Google's FCM) into one interface, so app developers only need to support this one client – and the user has the choice…
- Gotify (4.6@88) 5 1 Github; also allows you receiving notifications from your Nextcloud, Raspi, PC… via a Gotify Server (which you can also self-host)
- NoProvider2Push (0@0) 4 Github