The Wrath of the Lich King raid that was originally released as a 40-player raid in vanilla was Naxxramas.
The original Naxxramas opened in the Eastern Plaguelands during vanilla World of Warcraft as a 40-player endgame raid. It was the floating necropolis of the lich Kel’Thuzad and was widely regarded as one of the most demanding raids of the pre-Burning Crusade era. Its encounters tested coordination, resistance preparation, damage output, and precise execution.
For Wrath of the Lich King, Blizzard moved Naxxramas to Northrend and rebuilt it as an entry-level level-80 raid above Dragonblight. The raid retained many familiar bosses and themes, including Patchwerk, the Four Horsemen, Sapphiron, and Kel’Thuzad, but it was offered in 10-player and 25-player versions rather than the original 40-player format.
This makes Naxxramas unusual in World of Warcraft history: it was both the final major vanilla raid and the opening raid tier of Wrath. The Wrath version replaced the original, so the vanilla 40-player instance was no longer available in the live game after the expansion launched.