[psittacus]

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://github.com/lightningcpu/Alertly

[bembo]

What about this? https://f-droid.org/packages/co.adityarajput.notifilter