I think that's rather old fashioned, though I've never heard of this. Kids are always raised to eat their plates clean and etiquette changes over time, ie such wastefulness doesn't fit into today's social conscience.I had an etiquette book when I was younger . It said you should always leave some food on your plate for Miss Manners
I am surprised by such etiquette advice, esp considering all the wars of the past century and people barely making do... Though not everyone had rationing etc.
I'm waffling, anyway, quite interesting to learn!