FWIW, Breeze shadows can now be turned off entirely (System Settings → Application Style → Window Decorations → Breeze → Click on the little Settings button in the corner → Shadows → Size: None), so the only thing this really adds is opaque backgrounds. Not sure it's worth it IMHO. Maybe instead we should let people turn it off in Breeze without modifications.

FWIW, Breeze shadows can now be turned off entirely (System Settings → Application Style → Window Decorations → Breeze → Click on the little Settings button in the corner → Shadows → Size: None), so the only thing this really adds is opaque backgrounds.

You’re mistaken: this disable shadows only for window decorations. My themes disable shadows for panel (and widgets), Krunner and Kwin’s task switcher.

Not sure it's worth it IMHO. Maybe instead we should let people turn it off in Breeze without modifications.

Sure, you can add an option to use fallback resources; my solution is dead simple but a generic solution is better. I just don’t happen to have the time or knowledge to do it.

Also note that I use kquitapp5 plasmashell && plasmashell after applying my theme in order to make shadows disapear for Plasma; it works instantly for KWin.

Should I open a bug report on https://bugs.kde.org?

What bug would you be reporting?

To add an option to use the fallback of a Plasma theme (i.e. to disable shadows and transparency).

Yeah, if you abandon this patch, requesting that the feature be natively added would probably be sane.

I opened a bug but I didn’t have any answer so far.