Now that we are in the lead up to Battle for Azeroth, Blizzard has announced that all content through Legion is available without actually having to purchase the game. You will still need to, however, maintain a subscription to access all that content. In addition, and if you wish to take part in the Battle for Azeroth content, you will need to purchase the expansion.
The information comes from the Blizzard Shop. If you opt to purchase a subscription, you will see that you can battle to level 110 without having to buy the base game and / or a Battle Chest that used to contain all expansions up to the most recent.
Check out the World of Warcraft page on the Blizzard Shop for more information.