ntfy.sh | Send push notifications to your phone via PUT/POST is a self-hosted notification service that can push notifications to a smartphone. I know that HAS app can send notifications but sometimes I need notifications without HAS app installed. For instance I want some people receive notification without access to HAS installation.
I haven’t used it, but would this work?
https://ntfy.sh/examples/#home-assistant
Also, you can/ might want to vote for your own FR.