If you are someone who is still primarily on a Windows XP or Windows Vista PC (… in 2018, for whatever reason), you might want to upgrade your OS by the year end, especially if you have a substantial library of Steam games.
Valve has announced that it will be dropping support for Windows XP and Windows Vista in 2019- which not only means that the client will no longer support those OSs, but also that your Steam games will not be playable on them at all. Which is kind of a shame, since many games only work well on Windows XP.
The reason Valve cites for this move is newer features in Steam relying on an embedded version of Google Chrome, which in turn also does not support anything lower than Windows 7.
Now, I get the distaste of locking people out of their libraries unless their OSs are upgraded (especially since game preservation and continuity are often cited as benefits of PC gaming), but on the other hand- seriously. If you’re still on Windows XP or Vista, please upgrade. They’re super old, and super susceptible to virus attacks and security exploits.