I’m convinced the superstition is a misunderstanding over time of things that were, on their own, bad luck. Salt used to be expensive, so spilling some was bad luck because you would have rather kept it all for use instead of wasting it.

Mirrors would have also been expensive, especially when they needed to be transported before the time of smooth suspensions. The whole 7 years thing could be from it taking around 7 years for one particular broken mirror to be replaced.

Or the ones that invite accidents, like walking under a ladder (which usually implies someone is working at the top and might drop something, so odds of death are a bit higher under ladders). Or opening an umbrella indoors, where things are more crowded and you might injure someone or break something.

Though the black cat one is probably just racism.

Anyways, I bet that’s where they started and then humans being kinda (or very, depending on the circumstances) stupid and liking jumping on bandwagons they don’t always understand to fit in, left us with some people thinking those things cause ghosts to haunt you or whatever dumb shit superstitious people think happens.

Though I do think it is a bit wasteful to just dump salt out on the ground.