https://wccftech.com/blizzard-world-of-warcraft-pricing/

Blizzard has announced that World of Warcraft players will no longer have to purchase the original base game to gain access to the game and its previous expansions – all players will need is an active subscription.

Prior to this change, players were required to purchase previous expansions through the game’s Battle chest, but as of today, this is no longer required. In addition, In addition, Blizzard is automatically upgrading all existing World of Warcraft subscribers to Legion at no additional cost.