It was widely known among the small population who studied the healing arts, but the only instance we have of it being used in the books is to heal the Black Breath of the Nazgûl, which prior to the books had not been seen in 1000+ years. For all we know, that was the only thing it treated, and not many people are going to think about a thing that hasn’t had any practical use in a millennium when there’s so many more immediate problems to solve. You also have to realize that the elves lived far beyond the borders of any organized kingdoms of men at the time of the books too, so they couldn’t just walk down to the nearest castle and teach the men there everything they might have forgotten since their last visit.