Blizzard have announced this this weekend will be a "free" weekend for those who don't at that time have an active subscription:
Welcome Back Weekend
From March 21 through March 25
Return Now and Join New Allies
New allies have entered the fight for Azeroth, and you're needed on the frontlines. This weekend, we're upgrading all inactive WoW accounts to allow you full access to the game and all of your characters without a subscription. You’ll also be able access the Battle for Azeroth™ trial experience, even if you do not own the expansion.
Rejoin your guild, rally your comrades, and get back in the fight.
*Offer available through March 25, 2019. Player is responsible for any online connection fees. Game access will be granted at the last expansion purchased or World of Warcraft®: Legion®, whichever is greater. The Battle for Azeroth trial experience ends after three hours of game play in Battle for Azeroth, or after the quest "We Need Each Other" (for Horde) or "A Nation Divided" (for Alliance), whichever comes first. Players can repeat the trial experience with up to 12 characters per qualifying account during the welcome back weekend.