I gotta be honest, I'm not much a fan of the idea of a KS ban either. For a precedence comparison, Protean proved to be a factor in Greninja being broken, but we banned Greninja because he in particular was what proved unhealthy, rather than ban Protean despite its only other user being Kecleon. Similarly King's Shield is among the factors that made Aegislash problematic, but only as part of the whole. If we ban King's Shield to keep Aegislash, it does raise the question of why we didn't ban Protean and simply let Greninja drop to RU or wherever he'd end up.
My other problem is that this almost feels like it defeats the purpose of this suspect test. Aegislash works as a Blanket Check because he has the ability to abuse his mixed defenses on top of his typing. Without them, so many of his sets lose viability while others take a hit regardless. I only know of maybe 2 sets that didn't use KS and one that might work (if less effectively) without it: SD + 3 Attacks, 4 Attacks LO/Spooky Plate/etc., and the SubToxic set (w/ Protect). Without King's Shield, Aegislash is almost inferior in his role to Doublade, who has his Eviolite defenses consistently (Much better Physical and slightly lesser but usable Special) and still isn't considered an effective defensive answer to the above mons. I doubt this discussion would even be a thing if Aegislash could change forms with Protect, since his mixed defenses were always a bigger issue than the drops KS causes. KS doesn't let him toy around Lopunny because it inflicts the drops, but because of the 50% recoil and need for HJK to break his Shield Forme, which Protect would still keep intact. If Aegislash has to stay in Blade Forme after attacking, Lopunny can afford to hammer on him with a resisted Return instead, breaking SubToxic's subs and outspeeding offensive variants before a second Sacred Sword.
Now, what should set them apart is Aegislash's greater offensive presence, but in Blade forme, he's practically as frail as Sharpedo, and with no way to change back, he's forced to leave, making him a momentum sink and extremely vulnerable to Pursuit Trapping. The SubToxic set I noted is almost useless against the mons Aegislash is being used to check, because he lacks reliable recovery and is liable to keep getting hammered.
232 SpA Pixilate Mega Gardevoir Hyper Voice vs. 252 HP / 0 SpD Aegislash-Blade: 195-231 (60.1 - 71.2%) -- guaranteed 2HKO
252 SpA Life Orb Latios Draco Meteor vs. 252 HP / 0 SpD Aegislash-Blade: 238-281 (73.4 - 86.7%) -- guaranteed 2HKO
These are two mons Aegislash was known for putting a damper on while he was present, not because King's Shield could nerf them, but because his Shield Forme could stomach their hits easily before retaliating, and King's Shield let him take that form before any damage calcs could come up. Debatable how well Aegislash would even do checking the Megas he's being brought down for
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 0 Def Aegislash-Blade: 184-217 (56.7 - 66.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 SpA Mega Diancie Moonblast vs. 252 HP / 0 SpD Aegislash-Blade: 156-185 (48.1 - 57%) -- 91.8% chance to 2HKO after Stealth Rock and Leftovers recovery
So if Aegislash comes in to check them, they just have to switch out to the Pursuit trapper and Aegislash is essentially dead right there. A Pursuit trapper isn't exactly the most unreasonable support for mons even with such viability, considering Keldeo tends to really want it and he's in S-Rank. Heck, Diancie turns the tables on him because she can scout for the shift turn with Protect + 3 Attacks and force a 50/50 back on him, and the SubToxic set's only option against either is to Shadow Ball or Sub, both of which have exploitable consequences.
Heck, Pinsir, one of the more infamous cases of a Pokemon he inhibited isn't even really checked by Aegislash if he can't use his Shield Forme.
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 0 Def Aegislash-Shield: 174-204 (53.7 - 62.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252+ SpA Aegislash-Blade Shadow Ball vs. 0 HP / 4 SpD Mega Pinsir: 174-205 (64.2 - 75.6%) -- guaranteed 2HKO
4 Atk Aegislash-Blade Shadow Sneak vs. 0 HP / 0 Def Mega Pinsir: 54-64 (19.9 - 23.6%) -- possible 5HKO
252 Atk Mega Pinsir Knock Off (97.5 BP) vs. 252 HP / 0 Def Aegislash-Shield: 170-202 (52.4 - 62.3%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 0 Def Aegislash-Blade: 214-252 (66 - 77.7%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Swords Dance or Knock Off on the switch, Aegislash loses. Again, this is a mon he was infamous for giving trouble to, now setting up on him because he doesn't have to fear Shield Forme's defenses.
Losing the ability to change forms, irrespective of KS's secondary effects, defeats the purpose of Aegislash coming back. He can still switch into these mons... now what? He can't go on the offensive because the second he's in Blade forme he WILL get KO'd by any relevant attack and also becomes extreme Pursuit bait. Aegislash is a shaky check at best to most of the top tier mons at that point, becoming extremely passive if he's trying to maintain Shield Forme, and extremely vulnerable the second he goes to Blade. The passiveness and vulnerability to revenge killing make him suboptimal for offensive teams, while the lack of defensive consistency and control takes away the utility he offered to defensive cores. The 50/50's, if they're to be called such, around Aegislash for me never came down to whether I'd suffer a KS drop: the outcomes usually came more down to whether I'd be dealing with Blade or Shield Forme when I landed the next hit. If I knew Aegislash was stuck in Blade Forme once he attacked, he'd almost never register to me as a problem besides a Slaking tier wallbreaker: He gets on hard hit and then is the ultimate set-up fodder.
The matter of what we're trying to accomplish here is for discussion, but a ban on King's Shield really gets us nowhere. Aegislash is allowed back in fully, we get a more contained and centralized Metagame; Aegislash stays banned, the meta-game stays as wildly varied and unpredictable as it is now; No King's Shield, and the Meta probably sees a minor at best shift, Aegislash returning as a niche answer to these top offensive threats, though ultimately not holding them back if he can't consistently use his defenses. If a more contained Meta Game is the goal, the third option doesn't actually put us any closer than if Aegislash simply stayed banned, because he accomplishes the job he's wanted for by a sum of several factors that are all necessary for him to play as well as he does.
I'm still Pro Ban myself, but I'd say if the debate comes down to it King's Shield stays with Aegislash, wherever he's tiered. Aegislash's intent if we bring it down is to bring the tier back around it and reduce match-up issues. I feel like without the ability to use both his forms effectively, he fails to do that job to the extent he brings the metagame the Pro-Drop crowd most likely hopes to see.