Found at the mildlyinteresting subreddit.
These are called warded locks. If the key wasn't the same shape as the ward, the lock wouldn't open. Older versions were pretty easy to pick by modern standards, but wards are still in use. Modern locks have a plate on the front of them that defines the shape of key that the lock will accept. That's why you need to get a key cut from the appropriate blank or it won't work. If you check your keys, odds are very good that stamped on the bow (the part of the key that you hold to turn it in the lock) is a small letter/number code that identifies the blank and therefore the shape of the ward.Source: Former institutional (i.e. corporate) locksmith.
Warded locks are an old type of lock where the key has to pass over several obstructions (wards) as it rotates, before it can engage with the mechanism and unlock. It doesn't refer to the shape to which the blank must conform (the shape of the keyway), but the shape to which the final key must conform.
RacoonsinatrenchcoatYou are correct regarding warded locks using internal wards to stop a key from rotating unless it was the correct shape (unless it was a skeleton key, which bypassed these internal wards). But using that definition for a ward is too narrow. Wards are physical obstructions that stop the key from entering or turning the lock.If the ward stops the key from entering the lock, it's a keyway ward. In the lock, they're the protrusions from the keyway that necessitate the grooves that are cut down the length of the key. They partly define the shape of the blanks (in addition to other things like the number of pins in the cylinder). The cuts on the blade of modern keys are generally there to align the shear line on internal pins to open the lock and are therefore not associated with wards. Exceptions to this would be for things like control keys on interchangeable format cores, which allow the lock to be removed from the door with the simple turn of a key (great for swapping out office locks). I've also seen some modern padlocks, cubicle cabinet locks, and the like that use internal wards, but they're generally low quality.