That’s odd. I know highly literate people from all of the above, and I’ve never heard any of them misuse less.

Usually the highly literate folks misusing less are from Germany, the Netherlands or Scandinavia. It’s jarring when I hear or see it in otherwise erudite discourse.

Now, I know many native speakers of English in the US and various commonwealth nations that are unaware of the distinction. So I suspect you’re right on the larger point that this particular persnickety rule is dying in the vernacular everywhere. But I’d love to see your source for this rule not being universal in formal speech or when written.