Nintendo "intentionally held back" many aspects of Zelda: Tears of the Kingdom so there was more content for Hyrule Warriors to work with

Nintendo kept the Imprisoning War Sages in Tears of the Kingdom intentionally secret — director Hidemaro Fujibayashi says revealing them would've bloated TOTK’s story. Their faces/names were saved for Hyrule Warriors: Age of Imprisonment, where you can play them.