This is only my opinion, but King’s Shield on Aegislash is paramount for it to function effectively, even in-game.
To begin, Aegislash has two forms: Blade Forme and Shield Forme. Aegislash always starts a battle in Shield forme, with Defense and Special Defense base stats of 140 and Attack and Special Attack base stats of 50. Aegislash changes to Blade Forme (and the stats switch to ATK 140, DEF 50, SPATK 140, SpDEF 50) any time it uses an attack while in Shield Forme. Using status moves keeps it in Shield Forme.
The signature move of Aegislash, King’s Shield, protects the user (like Protect) and lowers the opponents Attack by two stages (like Charm), but only if the opponent uses a contact move. If these were the only effects the move had, you could for sure choose another move to replace it.
However, Aegislash is very easily KO’ed in Blade Forme. This I know from personal experience using Aegislash in Pokémon X. If the OP wants an Aegislash that they can depend on to tank hits and hit hard back during an in-game play through, I’d say King’s Shield is required.
Alternatively, unless I misunderstand Aegislash’s formes, one could simply use Swords Dance first turn in Shield Forme and then use Shadow Sneak to run through in-game teams. And if that is a strategy the OP wishes to employ, King’s Shield could be forgone in place of another move. But that’s only if the OP is ok with Aegislash fainting several times in battle during their game.
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