This is something that should have existed day one... of the initial release of FFXIV. There is quite honestly no genuine reason for this game to not have them.
When you put in the game items like bathing suits, pumpkin heads and most recently, the Lightning event gear... ALL of which have nominal stats at best and / or are low level... it makes it senseless to keep the gear because it has almost no viable use. What's worse is that the Lightning gear cannot be stored in the armoire... that's just poor Q/A there. You've basically just forced everyone who wants to do these quests for the gear to sacrifice 11 armory slots, 7 of which being weapons. Albeit, the storage issue is wholly separate from the appearance slot request, it is something that needs to be noted and recognized.