Makes it so you can't place water or hay bales while falling, so you can't jump off a cliff and just put down water or hay right before you hit the bottom to nullify fall damage. This makes falling dangerous again, and removes the incentive to carry a bucket of water or a bale of hay everywhere.
You can configure a list of items, then players right-clicking with those items won't do anything if they're falling. By default the list has hay bales, water buckets, lava buckets, cobwebs, slime, honey, ladders, vines, the other vines, and all 15 colors of beds.
Players also cannot get on a boat, minecart, horse, strider, or pig while falling. This is configurable. Note that boats still cancel fall damage, so I recommend getting Random Patches to fix that.
There's still the loophole of placing the water at the top of the cliff, waiting for it to pour down, then just jumping to the bottom, or swimming down the stream. Surface Tension by ElenaiDev fixes the jumping option by inflicting damage when you fall a significant distance onto water. The mod Fluid Physics by LolHens can remove the swimming option by making water source blocks move down, though I think you may be able to swim in the block while it falls down (haven't tested). My other mod Stationary Source Blocks removes the swimming loophole by blocking the placement of fluid blocks completely, though that also creates other gameplay challenges that your modpack will need to account for.