Informal usage has a history of it is as old as the English language. You can trace it back to Anglo-Saxon times and then you find it in Shakespeare, for instance. It's a huge puzzle to me to understand why the prescriptive grammarians of the 18th century decided to take against it in the way that they did. But the mindset of the time was such that they could only see that one kind of English was valued, that other kinds of English were not.