since at least 2019 there is f-droid privilege extension
to install you don't need root but you need twrp or similar custom recovery installed on your device. https://twrp.me/
you also need adb installed on your pc and usb debugging enabled on your device.
in the following steps all adb ...
If you want to "flex your root muscle" so to speak, there are options using Xposed modules, but these options do not require it.
Battery full notifications are a bit of a workaround:
Open Settings and select the Notifications option
Click See All under the Recently Sent list
Select All from the dropdown in the top left
Click the 3 dots in the top right and ...
Apparently I can't post comments yet, so here's my thoughts in an answer.
Try searching for the package name of the app that's showing the notifications. A quick Google search finds this app which seems to do what you're looking for.
Hope I helped.