Do you still enjoy Steam on your Windows XP machine? Seems that's just too bad for you, then. Valve's prior announcement that it will cease support for Windows XP and Vista comes into effect today. Users still on these platforms were still able to play games, but lost access to all other Steam features, in 2018; now, they are losing access to the client as a whole.
https://www.notebookcheck.net/Steam-support-for-Windows-XP-and-Vista-discontinued-from-today.388356.0.html
So, one question.
Why do they sell games that only work in XP or Vista?
Because every time someone asks this question on steamcommunity(dot)com he/she gets slayed by a resident Valve shill. Game devs are also to blame.