Well then, it was expected since the april's fool.
What to say, XY players must know how this pokémon is really centralizing and powerful.
Back in times I was against its ban but now things are changed.
Firstly, let's take an in-depth glance at it:
1- "potential" 720 BST considering its dual form that gives it (almost all times) extreme power when it attacks (150 atk/spatk) and extreme bulk when it receives an attack, due to its quite slow speed (60 Base stat) in an extreme fast tier as OU. Note that it can anyway outspeed several threats that would be dangerous for it, for example adamant Bisharp if you run max speed+ (240 vs 239).
2- god-tier typing, both in a offensive and defensive point of view. Ghost type has only 1 immunity and 1 resistance, both weak to Fight (and Aegi almost always run Sacred Sword for this reason), and also Steel type helps vs bulky fairies, that otherwise could in some way tank hits and statup (clefable) or do stuff (sylveon). About its defensive utility, Aegislash was one of the really few pokemon able to get off the meta a huge amount of really powerful mons as fights (medicham/heracross), fairies (gardevoir), ghosts (gengar), psychics (celebi/mew/slowbro/starmie/jirachi), dragons, flyings (pinsir forced to run EQ).
3- really optimal movepool, that can allows it to run several sets, with totally different features, as Mixed pivot (bulky lefties set or faster orb set, or also pursuiter; it can run also spooky plate or weakness policy), Sword Dancer, SubToxic or the rare autotomizer. It's necessary to be said that several counters of a certain set are blasted by others. As example: Chesnaught, wall of mixed pivot due to BProof, will lose vs SubToxic. SDef Heatran or Bisharp, "counters" of Subtoxic, are blasted by Pivot set. Sylveon, that can tank mixed w/out steel move and subtoxic (if runs HBell), is devastated by SDancer.
4- 50/50 argument. Damn guys this will be so toxic for the metagame, nothing else to say about it.
5- it limits teambuilding. I know this is usually a "ban" argument but, in order to have a "more skilled" metagame we have to reduce the matchup reliability in games. Aegislash, if reintroduced, will push out of usage several known as powerful threats such MegaAltaria, MegaMetagross, MegaDiancie, Latwins, Celebi, Starmie, Slowbro ecc. so the meta will be somehow restricted, so the matchup issue would be "fixed". teambuilding also will have negative downsides like, putting 2-3 ways to deal with Aegislash (for every potential set it should run)
im tired of writing so in short:
there's some reasons to say Aegi would be helpful for current OU tier, but there are also more reasons to say no. That's my thought atm, however I will ladder to clear my head on this
as I said for Giratina (even if that was a joke suspect), current metagame IMO doesn't need unbans but bans. We have a lot of threats to deal with for teambuilding, adding other ones, even if they restrict others, won't be as helpful as banning cancerous ones (e.g. Landorus-I, Mega Metagross, maybe more like Lopunny that will be overpowered by Aegi unban cuz no more solid counters viable lulz and some fairies)