This might be really naive, but aren't socially constructed phenomena real, rather than myths, because they're socially constructed? Because the social world is a real world that we live in? Even if almost nobody can conclusively state every single rule of Really Truly True Real Standard English, don't the facts of a million teachers with red pens, a thousand "whoms," and a general vague sense held by almost everyone who has gone to school that there is such a thing as a standard language make it at the very least an important thing to be reckoned with?
I don't think most people would disagree, in fact, that these are "important things to be reckoned with." I just don't think it's very helpful to use the word "myth" to describe them.
I remember when, as an undergraduate, I learned in a sociology class that race was socially constructed. I felt like I had just learned an amazing secret that hardly anybody else had access to. "Race isn't even real," I'd say, as if that somehow settled things. It took me a little while longer to realize what real meant.