This seems like a silly question at first, but I am not clear how this works with rules as written.
The 1st level spell Sleep has the keywords Arcane, Charm, Enchantment, and Implement. It previously had the Sleep keyword, but this was replaced with Charm in the Player's Handbook errata.
An Armor Guardian is a level 20 creature that is immune to disease, poison, and sleep (Published in Tomb of Horrors, page(s) 139, Monster Manual 3).
Would an Armor Guardian be affected by the Sleep spell? Would the spell function normally since it does not have the Sleep keyword?