To me anyway...
running touch today and marking 5m back from scrums (U11s in England FYI).
At half time one of the coaches - very politely - informs me that the 5m offside isn't enforced for the attacking side. I thought about it over half time and discussed it with my other TJ and the ref and we weren't sure TBH. So as I went back for the 2nd half just said to the coach ... "OK... but of course if they get shoved off the ball then they'd have to get back OK? " He seemed a bit nonplussed as he clearly hadn't thought of this possibility.
I've checked law 20 having got home now, and I was "right" - 5M offside line DOES exist for both sides as I was marking (and continues to mark the rest of the game as it happened anyway).
So - alert out for potential new myth!!