I've been running this in 3.8 for about 3 weeks now and everything is running great. One thing I never realized until tonight when doing some upkeeping is I had all of my wallpapers coming from an old ~ dir and I finally moved them.
The problem started here. I usually have a ton of wallpapers in Backslide and this was not good because as soon as I started moving those files, notifications started popping up, and they probably would have kept coming for hours if I didn't restart the shell. I possibly could have reloaded the extension but I didn't bother, but this is reproducible without much effort if testing is needed.
I think a simple a notification on/off switch would suffice. And/or if at all possible add an option to show one notification instead of all of them, when multiple pictures are detected as missing:
a) lets say for < 5 pictures missing, you get multiple notifications b) and for > 5 just 1 notification stating multiple files are missing?