This could be a reference to the undead plague mentioned in original Thief trilogy, or another plague altogether which predates it.
Strangely, the Bronzehead Bracelet (belonging to a widow whose husband was killed in the Plague) is inside of a modern Northcrest's Watch (which according to the game has only been founded recently) chest with the modern "W" symbol . It is possible though that the chest was brought down into the The Hidden City by The Graven. In which case, the Graven discovered the bracelet inside the catacombs or ruins, or it was brought down with the chest. In which case the Great Plague could have occurred at any point in time.
It is possible that the victims of the Plague were buried in the Catacombs where the chest is located (under the ruins of The Old Cathedral), 7-8 layers beneath the city. This may suggest that it predates the undead plague.