https://www.bleepingcomputer.com/news/gaming/steam-will-no-longer-support-windows-xp-and-vista-in-2019/

In what many would consider overdue, Steam will no longer support the Windows XP and Windows Vista operating systems starting on January 1st 2019.

Supporting older versions of operating systems can become a development nightmare, especially when components of your application rely on third-party applications and security features of newer operating systems. Such is the case with the Steam client that already relies on on an embedded version of Chrome that no longer works on Windows XP and Windows Vista.