https://www.polygon.com/2018/7/17/17583338/world-of-warcraft-legion-battle-chest-subscription-only

All of World of Warcraft’s available content — aside from the soon to be released Battle for Azeroth expansion — is now included in the game’s $14.99 monthly subscription fee. The Battle Chest — which used to include the base game and all previous expansions — is no longer available for purchase in Blizzard’s online shop. This change appears to be permanent.

Players, new and old, can now sign up for a subscription and play all the way through Legion, World of Warcraft’s sixth expansion. This change comes shortly after the pre-patch for Battle for Azeroth, World of Warcraft’s seventh expansion, which is scheduled to be released on Aug. 14.