You might be just as shocked as me, but yes, Sylvanas Windrunner is a limited-time Hero and it is going to disappear from the shop very soon. While I knew that Dame Hazelbark, a part Winter Veil Bundle (read more about it here) will no longer be available once the bundle is gone from the shop, I thought that Sylvanas will remain there for longer.
I’m not really sure how Blizzard’s monetization plans look like right now, but I would imagine that the goal is to offer Heroes for limited time so people will be more urged to buy them, make them unavailable, and then rotate them back in from time to time. Because I can’t imagine them making a Hero like Sylvanas just to sell her for only a month.
I also wonder if they have something else planned to take the Sylvanas’ place, because with Alleria, Magni and Medivh also out of store, it would mean no cosmetic Heroes whatsoever available for players to purchase.
Either way, it’s your last chance to get Sylvanas ($9.99) and Dame Hazelbark ($24.99, but comes with a bundle of 30 card packs) and their respective card backs. Both of them rotate out of shop tomorrow (January 6, 2020).