How to access a launch daemon's values/ system defaults value
Solution 1:
It's most likely not the answer that you're looking for, but suddenly I discovered a simple and trivial thing - "do not disturb" mode silences that notification, along with all the others of course...
I know that it's not a proper solution, but I'll stick with it by now. How could I've been so blind :D
Solution 2:
Try the following command:
defaults write com.apple.diskspaced removeAllNotifications -bool true