Notifications in mobile apps aren’t easy. A developer can’t just write code for the app to poll the server every x minutes. When the app isn’t at the foreground, it’s usually frozen and can’t execute any code. So, to get notifications, the app must rely on the host operating system and must have a server running somewhere to talk to the OS. Servers cost money, so now the developer must charge something from users, or sell their data, etc.
So, rejoice on the fact that Voyager is putting you in control of your data, ethically. And just check the app every now and then.
I think in Android you can have background tasks that do the polling and alerting. No need for a separate server, but you do need to disable battery optimization for the app.
If I were an open source developer coding for fun, I wouldn’t want to bother setting up a payment system and the whole nine yards that come with it. Sounds boring as fuck.
But hey, it’s open source, so why don’t you do it?