Not called any where near as much since the capt challenge came in as the refs know it will be challenged and overturned.When I played as a teenager a knock on had to be propelling the ball forward, if you were tackled and the ball came out behind you either one of your teammates picks it up or an opposition player dives over the top and steals it.
That was never a knock on.
Annoys the crap out of me these days as basically every dropped ball is a knock on EVEN if it goes backwards!!