Hello !
Usually on skins that are from a particular event it shows both on the skin list on the left with an icon where it comes from and also on the right, this time as a text, under the button EQUIP and UNLOCK.
Most of the skins follow that rule, exemple for Mercy the “Combat Medic Ziegler” skin has the (A) sort of icon for Archives and on the left the text says : “Available in Archives Loot boxes”.
The “Sugar Plum Fairy” skin however does show the icon on the left box list of skins for winter event but on the right hand part of the screen where it should say “Available in Winter loot boxes” : it doesn’t show anything…
It’s the only cosmtic item that has this bug (and hopefully it is a bug and this skin is still available comes next winter).
Thanks !