Of course, that doesn't mean Blizzard are making World of Warcraft free to play, because they're not - players still have to pay the monthly subscription fee but the company will not charge you for the game once you pay the subscription.
Before you throw your elven hat into the air, you should know that World of Warcraft's upcoming expansion Battle for Azeroth will still set you back a pretty penny when it launches on 14 August 2018. The expansion will go for $49.99, plus the $15 monthly fee, so you may want to plan your spending in advance.
However, the base game and the remaining expansions will all be purchasable for the $15 subscription fee, which is a pretty good deal all things considered. This includes The Burning Crusade, Wrath of the Lich King, Cataclysm, Mists of Pandaria, Warlords of Draenor and Legion.
If you really wanted to toot Blizzard's horn, you could add up the costs of the expansion and make the company look like Mother Theresa with what they're giving away for free. However, you would have to lobotomise yourself to forget that these expansions are eventually applied to all World of Warcraft accounts as new expansions arrive.
Nevertheless, getting into World of Warcraft has never been a cheaper affair, because $15 buys you everything but the latest expansion. Not buying Battle for Azeroth will probably make you the forever-alone kid of the bunch but either way - just sizing whether WoW is your cup of tea had been too pricey a test for too long.
As these things usually go, many have started to suspect Blizzard are leaking WoW players somewhat seriously. After all, this is not the company that's famous for being charitable, despite what Overwatch's may say.
Unfortunately, Blizzard have stopped releasing player numbers sometime in 2015, so we've no means to investigate these suspicions. We do know that WoW was at 5.5 million players at that time, but that's almost three years ago, in which time many things could've changed.