But it is extremely unlikely that any young person today has ever seen a hen have its head cut off

That depends on your definition of "young". This 33 year old remembers having to the legs of poultry while his older stepbrother decapitated them, and remembers being hugely entertained by watching them even manage to fly for up to 90 seconds or so afterwards. I suspect that the hateful job of plucking was karmic revenge for having laughed at the headless birds.