How is it used in the wild, in your experience? Have you observed projects following some alternate set of rules?

I thought projects that stay on 0.x.y forever mostly do it because it means they're allowed to break things. Also, since 0.x.y means "anything goes", projects can introduce their own conventions within that range without violating the spec.

I know that some package managers (including Cargo and npm) confusingly treat 0.1.0 → 0.1.1 like a "minor" update, despite the spec. Is this what you're referring to?