The base game and all six of the currently available expansions for World of Warcraft are now included in the game’s monthly fee.
Blizzard announced the change to World of Warcraft’s payment model on its blog along with the Battle for Azeroth pre-patch, which is available to download now.
All World of Warcraft subscribers have been automatically upgraded to the most recent expansion of the game, Legion, free of charge. This effectively opens all of the game’s available expansions for the entire player base – that is, until Battle for Azeroth comes out next month.
Source: IGN.com All Current World of Warcraft Expansions Now Included With Subscription