On Android we have:
1. Pokey that people can install to be notified and that seems to be working extremely well. 

https://github.com/koalasat/pokey

2. I made a quick push to generalize notification providers
 https://github.com/nostr-protocol/nips/pull/1528

3. Amethyst implements a super simple relay watcher and sends it via Google Play Services and Unified Push. 

https://github.com/vitorpamplona/amethyst-push-notif-server