Of the Three Sacred Treasures handed down in the Imperial Household, the Kusanagi sword alone should have been lost, sunk into the sea in the Battle of Dan-no-ura.
Of the Three Sacred Treasures handed down in the Imperial Household (the Japanese emperor’s regalia), only the actual Kusanagi-no-Tsurugi was, as I recall, supposed to have been lost, sunk into the sea in the Battle of Dan-no-ura (1185), but as of now it is said that the thing that sank at Dan-no-ura was a replica called a katashiro (a substitute/effigy).
This case is part of the Mandela Effect case database based on "The Window of Opportunity: A Guidepost to the Mandela Effect — The Ultra-Secret X-Files" by Aoyama Takuto.