The Flight Master’s Whistle obtained during Legion is usable in Battle for Azeroth once you unlock world quests. If you didn’t have one, you will receive one.
When you unlock Battle for Azeroth world quests, your Flight Master’s Whistle gains the ability to work in the new lands. if you didn’t play Legion or destroyed your whistle, you will receive a Flight Master’s Whistle that works in Legion and Battle for Azeroth zones.
— WarcraftDevs (@WarcraftDevs) August 17, 2018