Just a little correction to the flaw in accepted answer, that great in every other aspect.

Monk's Slow fall

Beginning at 4th level, you can use your reaction when you fall to reduce any falling damage you take by an amount equal to five times your monk level.

Xanathar's Guide to Everything

If you’re unsure when a reaction occurs in relation to its trigger, here’s the rule: the reaction happens after its trigger completes, unless the description of the reaction explicitly says otherwise.

The Monk's feature requires the use of reaction during the fall, but the effects of that reaction removes damage monk takes after the fall, when the fall damage is applied. Effect is worded in the same way as emphasized in every other feature mentioned in answer. So I would assume that it would work the same way.

Just a little correction to the flaw in the accepted answer, which is great in every other aspect.

Monk's Slow fall

Beginning at 4th level, you can use your reaction when you fall to reduce any falling damage you take by an amount equal to five times your monk level.

Xanathar's Guide to Everything

If you’re unsure when a reaction occurs in relation to its trigger, here’s the rule: the reaction happens after its trigger completes, unless the description of the reaction explicitly says otherwise.

The Monk's feature requires the use of reaction during the fall, but the effects of that reaction removes damage the monk takes after the fall, when the fall damage is applied. The effect is worded in the same way as emphasized in every other feature mentioned in the answer. So I would assume that it would work the same way.