The Primarch Magnus knows it or purports to, when he speaks to the Khan when JK is visiting the ruins of Prospero. The new Abbnett series about Eisnhonr v Ravenor also floats it as a plot device, to introduce the Word Bearers as a faction in the novel. They are apparently trying to use Enuncia to learn it so they can do something funky to Him who be-eth on Earth.