Why on earth do people think this is a thing
It's been a thing since before the internet was invented, let alone Mumsnet.
Before meat was mass produced and relatively cheap, the only meat a family would get in a week was the Sunday roast, which was stretched through leftovers for a couple of days of weekday meals. Beyond that, people would have things like egg and chips as their main meal.
But chicken was far too expensive to be affordable so people would generally have other meat like pork or beef instead. A chicken would be something people would have once or twice a year as a massive treat, it was not an expectation to eat relatively large portions, several times a week like it is today.