It took me a moment to find, but Alertly claims to do something similar while being open-source. Last commit was made two years ago though. https://f-droid.org

It took me a moment to find, but Alertly claims to do something similar while being open-source. Last commit was made two years ago though. https://f-droid.org/packages/com.example.notificationalerter https://f-droid.org/packages/com.example.notificationalerter https://github.com/lightningcpu/Alertly https://github.com/lightningcpu/Alertly