Battle for Azeroth Pathfinder, Part One has been added to the game and the achievement reveals the first part of flying requirements in Battle for Azeroth.
Flying in Battle for Azeroth
Similar to Legion, flying won’t be accessible right away in the new expansion and you must complete two Pathfinder achievements to be able to fly. The first part can be completed right away, whereas the second part will be added to the game later.
Battle for Azeroth Pathfinder, Part One (Alliance)