In a post on the World of Warcraft forums, Blizzard writes, “Starting later this year, we will begin the process of ending support for Windows XP and Windows Vista in World of Warcraft, StarCraft II, Diablo III, Hearthstone, and Heroes of the Storm.” They also added that because a good many players still used those platforms, Blizzard decided to continue their support for the time being.
However as there have been at least 3 major Windows releases since Vista, they believe that it is time to move on. “After these older operating systems are no longer supported, the games will not run on them, so we encourage any players who are still using one of the older OSes to upgrade to a newer version. We’ll be rolling out this change on a staggered schedule, and will post further notices as we get closer to making the change for each game.”
This won’t go into effect right away and as Blizzard states, it will only come into effect later this year and it will be done so in a staggered manner. So if you haven’t upgraded already for whatever reason, perhaps now’s a good time to do so.