I know this doesn't fit your criterea OP, but if anyone else is looking for some kind of notification service, I use: SMTP to Telegram
I get instantly notified on my phone for healthchecks.io failures, cronjob reports for different scripts like borg backups or ddns update failures, certain Home Assistant scripts, and Sonarr completions so I know when a new TV episode is done downloading, and a bunch of other things set to notify on failure like SMART failures or snapraid-runner failures or distro updates... so many things. It's nice having peace of mind that if I haven't been notified that something is wrong, then I know everything is working, and I do not need to check on it. So it's one of my favorite services that I'm running.
I don't think I need to say it, but this is obviously not something you would put facing WAN as there is no TLS nor authentication.