Since 12.0 there is the ability to receive push notifications on (mobile) clients. I was testing the Android 2.0.0 client and successfully received a push notification.
I just noticed in my NC logs there is a message about
https://push-notifications.nextcloud.com/ and I was wondering what exactly this server does, why it’s needed etc? I found https://github.com/nextcloud/notifications/pull/59 which refers to https://github.com/nextcloud/pushnotificationproxy, IMO it would be very good if this server would be open-source too.