As others have said, the unmarked “zone” on the part of the EK map is occupied by uninstanced Stratholme. Well, partially anyway — you see all that “smoke” above Strat in Eastern Plaguelands? That’s masking two things: a part of a Naxxramas prototype that never got removed from the Stratholme model and a gigantic empty, flat void behind it. It wasn’t uncommon for these kinds of voids to exist back before flying was added to Azeroth, but most of those were fixed in Cataclysm. Only outdoor Stratholme was left unfixed.
Why did they leave it unfixed? Well, as you might know, the Blood Elf areas (Eversong and Ghostlands) that were added to Eastern Kingdoms in The Burning Crusade aren’t actually part of the Eastern Kingdoms map. Instead, they’re part of the Outland map, presumably to prevent people who hadn’t bought TBC from accessing it.
When the Ghostlands was being made, the map artists designed as they pleased without regard to space constraints. This worked because the map was separate, but it means that the southwestern part of the Ghostlands (known as Deatholme) technically occupies the same space as Stratholme! Because of this, merging Eversong + Ghostlands into Eastern Kingdoms proper was greatly complicated (at minimum, they’d have to move the entirety of either Stratholme or Deatholme), and Cataclysm’s development already had the dev team stretched thin, so they elected to put up and invisible wall and cover the Stratholme void-area up with smoke.
If you find this interesting, can read more about this and see screenshots demonstrating the overlap between the two maps here.