According to Frayling, Clint Eastwood said: "You had to shoot separately, and then show the person fall. And that was always thought sort of stupid, but on television we always did it that way...
Back to movies. Recently watched Gun Crazy (1950) and This Gun For Hire (1942). Both broadly follow the Hays guidelines. For example in Gun Crazy a boy shoots a chicken using a BB gun. The boy firing the gun is in one frame followed by a still of the dead chicken in the next frame. In This Gun For Hire there is even a closed door between the shooter and shootee. Alan Ladd fires a gun at a closed door behind which there is a woman. You never see the woman being killed but hear the noise of her falling to the floor.Both scenes are probably more effective than if they had been filmed with the shooter and the shootee in the same frame.
I never assumed that it was made up by them, but there are much too much examples which prove that this rule wasn't a rule in the decades before. So when it ever was a rule or some kind of an unspoken law, the question is when (if ever) was it an rule?And if it was a (written or unwritten) rule still in the 50s, why could so many films ignore it?
I don't know where that came from either, it may have been more a 1950's television convention at the time, it sure is a topic that needs a bit more investigation.