In order to earn the Battlemaster achievement, you have to complete the master achievements for Warsong Gulch, Arathi Basin, Alterac Valley, Eye of the Storm, and Strand of the Ancients. When Battle for Azeroth came out, Strand of the Ancients was removed from the game. However, the Battlemaster achievement was not moved to the legacy tab, indicating that you can still attain it.
If I did not complete the Master of Strand of the Ancients achievement before the battleground was removed from the game, does that mean I can no longer complete Battlemaster?
dly's answer lead me to realize that there is a difference between the Horde and Alliance versions of the achievement. The Horde version of the Battlemaster achievement still lists Strand as a requirement, while the Alliance version does not. Since it was removed from the Alliance side, I will assume for now that its presence in the Horde achievement is a bug.