The reason that our grandmothers would not have spoken about minimum square footage requirements is because they knew BY EXPERIENCE, and by eye, what you likely could and couldn't get away with.
If you didn't grow up around chickens, something a little more concrete is helpful.
Also, in older times people were much less hung up about reducing the flock if crowding-related problems developed... a higher proportion of people these days would rather avoid having to cull birds just because of having guessed wrong about how many they could keep.
Unfortunately the unpalatable bottom line is: the more space you can give the chickens, the better off they will be and the less likely problems will occur. The problem is, this is is in direct conflict with the fad for collecting all the collectibles you can, or with attempts to maximize production of eggs or meat with minimal expense/work. All the square footage numbers are just attempts to figure out how close you can likely cut it.
JMHO,
Pat