How to get a phone notification if my VPS goes offline?

https://slrpnk.net/post/35723833

How to get a phone notification if my VPS goes offline? - SLRPNK

What are my options for getting alerts on my android phone if services on my VPS becoming unresponsive? My first thought is a simple app that periodically pings domains and gives a notification if any fail. Is there an open source app for that? Or something else?

Ntfy can send/receive notifications to/from the phone. You can selfhost it or use a public instance. For the healthcheck app, consider Uptime Kuma as it has ntfy integration. But a simple cron script that monitors + cURLing ntfy when it fails could also be used.
ntfy

Send push notifications to your phone via PUT/POST

Ntfy is a good lightweight option indeed.