It's far from useless https://en.wikipedia.org/wiki/1986_California_Proposition_65#Accomplishments

It forced companies to reformulate their products to use less harmful chemicals to avoid having to use the label. And it was highly successful at that, not only in California but outside, because businesses don't want the labels on their products. Especially food products or products meant for kids.

What you see is malicious compliance from businesses as there is no penalty for putting the sign if there's no dangerous chemicals. If they put it often enough, then most people think this law is ridiculous. For example. If Disney would get a civil lawsuit (this is how the law is enforced), because for example one their restaurants were using dangerous chemicals, this sign won't protect them. So it serves no other purpose than to make it appear that the law is pointless.

Other ways they fight it is trying to pass federal law banning it, they had several attempts.

They also making strawman lawsuits, even creating companies specially for the lawsuit to show that this law hurts business.