That's interesting. Why was it considered so unimportant?
I'm not sure, but I think it was to encourage children to be creative rather than concentrating on parsing sentences and so on. I was taught grammar, and I'm 64, so it was after my school days, assuming it was rolled out across the country all at once.
I think it was fairly soon after 'my time', and they brought in a different way of teaching reading and writing at the same time, called ITA, which was loosely based on Pitman shorthand. It was responsible for a lot of children falling well behind, I think.
I have certainly taught very able students who didn't know what a subjunctive or even an adverb was, because nobody had told them, their parents or their teachers - it's not the sort of thing you learn instinctively, is it?
Regional differences are not necessarily 'wrong' either - just non-standard. Many regional speech patterns are closer to Middle (and even Old) English than the standard patterns used today. The fact that one type of speech pattern became 'Standard English' is entirely based on social, rather than linguistic criteria - the regional ones are every bit as good at conveying meaning.