I've actually heard people argue against having lights on signage for this exact reason: people shouldn't be reliant on lights that may or may not work to modul

I've actually heard people argue against having lights on signage for this exact reason: people shouldn't be reliant on lights that may or may not work to modulate their behavior when driving. They had been referring mainly to pedestrian crossing signs, but I think it applies here too. I generally treat any school speed limit sign as in effect if it's before nightfall as a rule of thumb.