This happened. It's part of the protections of Ondar Shambal and only effects Enemies of Creation (that is, Creatures of Darkness and Fairfolk), but it's happened.
And that's
not just a god's declaration. Ondar Shambal was built from the ground up in the First Age as a holy city. Such an effect will be anchored into its very geomancy and in the layout of the prayer city.
It's not something which can be done on the fly.
But yes, one the one hand plague is a great way of killing cities in Creation. On the other hand, Whitewall is not the kind of opponent you are going to defeat with some festering cattle flung over walls and rats dropped into wells.
Oh no, you have Abyssals. You're not going to be as crude as to toss some dead cows into their city.
With a combination of Abyssal Medicine and Abyssal Stealth, you have an asymptomatic carrier of a Virulence 4 disease who no one can notice (and who is optionally Outside Fate so Fate can't be used to track him down and arguably that means that Destinies can't be used to stop infection coming from him). From the description of plague in the corebook, it describes that plague comes as pneumonic plague as well as bubonic plague, which means you can have aerial transmission. Your unnoticeable, highly infectious, aerial transmission carrier can go everywhere in the city and infect anyone they feel like. They can go to the lowest slums and the highest dining places, and infect rich and poor alike. They can infect the healers so they'll unknowingly spread the disease. They can infect the barracks of the soldiery so they can't be used to maintain civic order. They won't be caught. By spreading it wide and getting lots of Patient Ones (the Abyssal is of course Patient Zero) the quarantines and district-lockdowns that they'll have to impose to stop it spreading will cripple the city even if they manage to contain it. You make sure to infect people leaving the city, so Whitewall ends up isolated because people hear it's a plague city and so don't come to trade with it.
Unlike normal cities, the traders can't even just stay outside a plague city Whitewall. You need to come into the walls to be safe. In its text, plague is explicitly described to be feared second only to the Great Contagion, so just hearing there's plague in Whitewall will isolate it.
And plague is Virulence 4, Morbidity 5(4), Difficulty to Treat 4. Most healers aren't any use for treating a disease like this, and are at more risk of catching the diseases and dying from it themselves than they are of actually saving people (remember, most professional healers will have a dicepool of 4-6 dice and aren't heroic, so most people can't be treated, and the ones who are treated will probably die). Whitewall is heavily overcrowded, and that won't help matters either. The way to fight such an engineered outbreak which takes far less effort to infect a new person than it does to cure one would normally be to quarantine off streets and let it burn itself out... but our invisible Outside of Fate Abyssal can just bypass the quarantine lines and even more amusingly infect the people who are maintaining the quarantine, so they'll wind up scared to enforce quarantine once they find out how their comrades are falling sick.
Now, yes, this won't make Whitewall fall all on its own. But it will cripple its internal governance, isolate it, and force it into self-damaging containment protocols. Against an invisible and asymptomatic Patient Zero who's deliberately spreading the disease, shit is going to go Masque of the Red Death on you.
(I would totally have that happen at a great party held for the highest in society if I was running it as an antagonist's ploy. It'd be a fun morality ploy for a certain kind of player who'd consider saving the poor infected more important than the wealthy who were having a party when there was plague.)
This is just using canon Charms with an Essence requirement of no more than 2, by the way (Plague-Eating Kiss for "I'm a plague carrier", Unseen Wisp Method and Atrocity Without Witness for "I'm outside Fate and you can't see me"). By the precedent set by The Green Mile in Malfeas' set, we know it's acceptable charmtech for Solaroids to have wide-area "infection fields" and so we might expect to see something like that in Abyssal Medicine where entire areas become foetid and stinking and no one can venture in and spend time inside without falling ill. If I was really building a focused Abyssal "plague doctor" character, I'd talk with my GM about charmtech and mechanics I can steal from Malfeas' Green Sun Wasting set.
