While equippable by the primary shield-user GLA/PLD, mages have been overlooked.

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Steps:
1. Note the the Emperor's New Shield is only equippable by GLA/PLD.
2. Note that functional shields up to Lv50 are also equipped by CNJ/WHM and THM/BLM.
3. Note that as a result, gladiators can hide their shield but mages cannot.

I assume this is a legacy of the course that the Emperor's New Shield has taken to come into being: from specific Lv60 GLA/PLD gear as part of the Garo collaboration, to a Lv1 replica of that GLA/PLD gear, to the Emperor's Shield which is no longer connected to Garo but is otherwise identical in being sold at the Gold Saucer for the same price.

If there was a conscious decision to not allow mages to use this shield, then that is disappointing but understandable. I realise that in the current state of the game, mages no longer use shields beyond Lv50 - but not everyone is above that level, whether as new players or alts, and the combination of wand and shield remains valid equipment in the earlier stages of the game.

I hope that the lack of availability to mages is simply an oversight, and that you can consider updating the range of classes permitted to equip the shield, and thus be permitted to use it as glamour.