Elytra Slot

Elytra Slot is a mod that uses the the Trinkets API to add a new slot, the Cape slot, to the player inventory and allows the elytra to be placed

As for your plug-in request itself, it's not possible to add an extra armor slot which will function as an elytra slot without a mod However,

elytra slot mod Elytra Slot - - minecraftmc-modselytra-slot

fat168 slot Trading · Elytra · Brewing · Villager Curse of Binding is an enchantment that prevents removal of a cursed item from its armor slot