Blizzard has taken stock of player feedback about the World of Warcraft achievement called "Defender of the Broken Isles". This achievement was part of the "Pathfinder Part 2" achievement that would unlock flying on a player's account.
The achievement required players to complete a demonic Assault in each of four regions. The issue came as a result of the "cyclical Assault" schedule that would make them appear at random times for limited duration. As a result, many players have been unable to complete this step in the overall progress to gain flight in Legion.
Developers’ notes: The rotating Legion Assault schedule is designed to have them active at varying times of day over the course of a given week, to accommodate different work schedules and available free time. We're hesitant to have an Assault last for too long, because they stomp on the entire World Quest content of a given zone for the duration, potentially leaving players with nothing to do there once the Assault is complete. But this design doesn't mesh well with an important achievement that requires completion of all four zones - if anything, it causes stress over scheduling and potentially missing specific zones during the cycle. The Defender achievement was never intended to be a major obstacle on the path to flight in the Broken Isles; its inclusion was more for thematic purposes and completeness. As such, we're removing it from the meta-achievement required to unlock flying.
The removal of this portion of the achievement removes one more roadblock to flying.
You can read the full hotfix notes and player reactions to the changes on the World of Warcraft site.