Do you mean the fur mantle? That was never made available to players. It was in CoV beta, removed, and never re-added. Same for the old bandage face-wrap costume part that is only used on NPCs. That, however, has a spiritual successor with the new Mummified set (which was planned to be Halloween content before the game got cancelled).
Nothing is missing from the costume creator. We even have NPC-only parts (Warp backpack, Freedom Corps pieces, Legacy Chain helmets, the new Circle of Thorns set, Banished Pantheon auras, etc.), everything being worked on for I24 and beyond (The Mummified set, Floating Head, Bodysuit sets, Tech Knight set, Post Apoc set, etc.), and previously unlockable-only parts (Ruluarru and Rikti weapons, all capes and auras, the Ascension set, everything that was once DLC).
If you have screenshots, please provide them and I can help you identify which costume parts you may be thinking of.
EDIT: I just checked Icon. It actually has fewer things than I25.