Not a grammar change but involving punctuation.
The use of lower case "s" and an apostrophe when pluralizing numbers and acronyms (e.g., F-14's, PC's).
I am told that current usage says leave out the apostrophe.
Although I may have used it incorrectly throughout my early school years, I was never corrected. Logically, the apostrophe connotes a possessive; but until recently, I thought when pluralizing things like "the roaring 20s", it was optional or at least acceptable. Did something change, or did I lead a sheltered life in grade school?