(And the OED doesn't say it is incorrect. I prefer the grand-daddy of the dictionaries to its filleted offspring.)
Having said that, I totally agree with most of your latest comment. I, too, would not dream of using 'off of' in a formal - or indeed any other - situation. The latter is purely because it was not used in the area in which I was brought up and nor has it been much used where (and how) I have lived since. The phrase is simply not part of my day-to-day vocabulary.
My point, however, is that many people were brought up in circumstances where the phrase IS a normal linguistic usage and - perfectly naturally - they continue to use it.
For my part, I can see no reason whatever for them to cease doing so or to be told what they are saying is 'wrong'. For them, it isn't and they have no interest in the fact that you and others imagine it is.
You appear to concur with what I said about 'context'. I assume you are happy with txtspk when you are texting, as am I, but neither of us would employ it in an academic or formal submission of any sort. Thus, txtspk isn't 'wrong' either in any kind of general way.
I think we'd better just leave it there, before I get started on the Rolling Stones' song, "Hey you, get off of my cloud!"