The bug in question, according to the patch notes, “Fixed bug where in-game overlay could take two screenshots for a single hotkey press in some games that process input via DirectInput and Win32 input messages.” Basically this caused two screenshots to be taken instead of one. It doesn’t sound like a particularly big deal, but it does eventually add up to a lot of unnecessary wastage of hard disk space, especially if you don’t pay close attention.
That being said, the bug has been around for years now with some spotting it way back in 2011. It is unclear as to why Valve took so long to do away with the bug especially since it seems like a relatively easy thing to fix, but perhaps due to the fact that it isn’t exactly a system breaking bug or a security risk, Valve did not prioritize it and it kept getting passed over for more serious issues at least until now. We suppose better late than never, right?