While not self hosted, I have been really happy using Pushover for simple and OS vendor decoupled alerting.
Funnily enough, the author of pushover is the same guy that originally built lobste.rs.
I was going to go for it but I probably need about one or two notifications a month and I couldn’t find a copy of the app on f-droid. If I could’ve got the app on f-droid I’d have probably gone for it.
I don’t think that the app is free software, so it can’t go into the main repository of F-Droid.
I love pushover too! It’s silly easy to use which makes it dead useful. For example, if I’m running a long command I like to wander from my desk to stretch my legs, so I have my shell notify me when any command running for longer than 2 minutes finishes. And I’ve used it for “monitoring” non-prod stuff, when I don’t want to bother setting up a full blown logging / alerting stack (which is often).