This is a new standard for BA. I don't think it existed in 2015 or at least they hadn't gotten to enforce it yet. I think if your bag is soft and not stuffed, they might not tag you. However, in the past they never paid attention to personal items and this year they are. If they are close to being too large, they put a yellow tag on it so you can get it through the gate. The standards for surveying this were more strict in different airports. Their purpose is to make sure you keep your personal item under the seat. They don't want any in the overhead bins, so think of your size and your leg room, too, when choosing your bag. Mine was 2" smaller, if I recall correctly, and I found it to be perfect, especially for aisle seats that often have an entertainment box.
PS: This observation was from my trip to Greece, which ended two weeks ago. Obviously, they don't tag your bag if you check your suitcase. I did carryon, on the way, which I loathe to do, but I had a sleep over between flights so thought this was best, so I needed my personal items perused, and I received a yellow tag. On the way home, I was still watching the airline and their yellow tags. Even though the flight was not full, they were still yellow tagging the personal items, for those who had a carryon as well.