That soft, and velvet-like muzzle we love to nuzzle isn’t why clipping a horse’s whiskers is a bad idea. The trimming of a horse's whiskers has been banned in Germany and Switzerland by law (animal welfare act) for many years.
It is considered as cruelty to cut or shave them off for the whiskers help the horse to sense/feel an object (for instance when picking up food from the floor etc. or in the dark) before actually touching it hence preventing injury (especially to the eyes).
In the midst of the muzzle’s soft skin are scattered, course hairs, called vibrissae, or tactile hairs. These whiskers are a result of millions of years of evolution to help horses sense their environment.
Cutting those hairs can lead to injuries as a result of horses failing to avoid obstacles near their faces. Because horse’s eyes are set on the side of their heads. they have a blind spot in front of and right under their noses.
So, their whiskers act as a sort of “third eye” helping them to negotiate objects close to their noses, such as food, water and the ground, avoiding injury and making sure they don’t eat the wrong thing.
Eurodressage did an extensive article on the subject that has gone viral: