[Reminder] As Valve had anticipated in the middle of this year, its digital distribution service will no longer receive updates for Windows XP or Windows Vista as of January 1, 2019.
This means that after that date, the Steam Client will not run on those versions of Windows. To use Steam and any game or product purchased through Steam, users will have to upgrade to a newer version of Windows.
The new features of Steam depend a lot on the integrated version of Google Chrome, which no longer works in previous versions of Windows. In addition, future versions of Steam will require security updates and Windows features that are only present from Windows 7 on-wards.
Steam will continue to run games on Windows XP and Windows Vista for the rest of 2018, but it is possible that other Steam features are somewhat limited. For example, new features such as the new Steam chat client.