A mission table in BFA stated Raleigh the Devout took a group of paladins to cleanse the blight and were relatively successful. According to the Exploring Azeroth series, the Alliance control Southshore and are cleaning up the remaining pockets of blight and threats.
How actually? No clue. Thinking Blizzard forgot they blighted it for 100 years into the future and realized it was a mistake having this Horde exclusive contested zone that was a beloved zone prior to Cata when they attempted to balance the questing options for the two factions.