In June Valve warned users that the most up to date version of the Steam Windows Desktop client would no longer be compatible with Microsoft's Windows XP and Vista, but magnanimously extended a grace period for users on a legacy client version until the end of 2018. That time has now arrived, and from tomorrow the legacy Steam desktop client will not be supported any longer. Not only does that mean a minority of users will be be unable to purchase software through it, but also that they'll no longer have access to games linked to their Steam account until they update their Operating System.
The current version of the Steam Client makes use of an embedded Chrome web browser that itself not longer supports older versions of Windows. The change in policy is therefore ostensibly necessary to ensure the broadest possible support and higher levels of account security, all the while removing the legacy client as a vulnerability.
When the policy change was announced approximately 0.3% of Steam Hardware Survey respondents had Windows XP installed, and fewer still were running Windows Vista. Six months on and the proportion of users affected by the change should be even lower. If you are affected, it may be worth scouting out a bargain upgrade in the New Year Sales.
SOURCE: Steam Support