In the season 4 Hashira training arc, Amane Ubuyashiki tells all the Hashira about the marks. It is the first known mention of the phrase "demon slayer mark" in the anime.
It’s also why the Hashira training happens - the Hashira want to develop the mark due to the advantages it offers after learning it is only accessible during incredible physical exertion. AKA, the bulging veins and eyes from Tanjiro we’ve come to love.
