I think they would probably use sharding for the AQ event.
And why not?
Back in Vanilla events like that were characterized by glacially slow framerate, loot lag, and server crashes. All caused by overcrowding as a majority of the online playerbase crowded into just one zone. The game couldn’t handle it.
And even when the game was actually playable it wasn’t very fun with so many people around. Mobs were tagged instantly and quest items snapped up. It was virtually impossible to actually get the quests done. It was downright frustrating.
Sometimes Blizzard tried to fix this by speeding up mob respawn. At its peak it was rather comical. literally the instant you killed a mob it respawned. This sometimes made it very difficult to loot since you couldn’t exit combat.
And was that a great gaming experience just because it was all on the server? … absolutely not. It was frustrating. It was painful. Many players logged out in disgust. People were not having fun and there were tons of complaints from angry players on the forums.
Sharding would fix all that.