Those unit sprites are directly from the 3.23 game data, as far as I am aware. Can you tell me the unit name or number for a "new" Longdead sprite in the game and show them on a screenshot, maybe? I don't see any difference. According to the
Illwinter changelog, the last time that the Longdead sprites were "niceified" was in version 3.16.
Have something else in the meantime: Here are the "lost" sprites from KO's scribble book, preliminary versions of sprites that weren't finished yet or where KO was making multiple versions of a unit's sprite until he found out which one he wanted to use in the game.