You won’t be able to play Blizzard games on Windows XP, and Vista starting this October.
The forum post noted that Microsoft itself ended support for both OSes in 2009 (XP) and 2012 (Vista), but Blizzard didn’t when it saw that a number of players were still using them at the time.
This is really more of an issue for older games, since Overwatch does not support these OSes, for instance.
Blizzard actually revealed plans to end support for the two OSes back in February, but it posted an updated note this week to remind players of the upcoming change.