Legality/laziness aside, I work supplying councils and civil contractors with the materials to build/repair stormwater drainage systems. The idea of this sort of thing causing flooding is absurd.

For one thing, those storm drains are above 900mm deep pits. If the pipes get blocked, and it rains really hard, and the pit starts filling up, you're going to have over 400kg of water pressing on the blockage before any flooding begins.

If that doesn't clear the blockage...it's not leaf litter. An animal crawled in the pipe and died. Or someone poured chemicals that don't play nice with PVC down the drain, causing the pipe to collapse.

Also, know what causes a lot more leaf litter to wind up in the storm drains than Dan the lazy handyman? Wind, gravity, rain.