Most companies have already abandoned Windows XP and Vista, including Microsoft once Vista leaves extended support in April, but Blizzard is known for long-term support. This is the company that is still selling Diablo 2, even producing retail disks for it last I checked, almost seventeen years after it was released (including a patch last year).
Later this year, World of Warcraft, StarCraft II, Diablo III, Hearthstone, and Heroes of the Storm will no longer support Windows XP or Vista. This will not all happen at once, even though it would actually make less sense if they did. I mean, why would they coordinate several teams to release a patch at the same time and maximize annoyance to the affected users who cannot schedule or afford an upgrade at that specific time?
Although, if that’s you, then you should probably get around to it sooner than later.