The regs probably mean 1/4 the length of 1 leaf in the stack.
A long time ago most trucks had leaf spring suspensions like Reyco and Hendrickson.
It was common to see broken leafs in the spring stacks.
If a broken leaf made it’s way out of the stack it could cause a lot of damage to your truck or other vehicles, blow a tire, rip a fuel line, damage your air brakes, smash through a windshield, etc.
After time the steel sort of crystallizes, becomes brittle and breaks.
They could break a few inches from the ends to the centre of the stack or anywhere in between.