It looks like sometime in the past couple of months that the Steam client in Linux started including all Windows games in the penguin icon. Previously this was only Linux games.
I'll need to go back and delete all the Windows games downloaded via the Steam client in Linux since I already download them all in Windows.
What really sucks is when you use the console commands to specify to only download for a specific OS then ALL games are shown so you don't know which games are for which OS except for when you try to install and it errors out.
I've used these in the macOS steam client for years, so will likely have to do the same for Linux:
Go to the Console tab and type the following commands (and press Enter at the end of each):
a. @sSteamCmdForcePlatformType macos (where type is windows, macos, or linux)
b. @sSteamCmdForcePlatformBitness 64 (where bitness is 32 or 64)
So if you were in a situation where you have 600+ steam games that have never been installed and you specify Linux only via the console then you'd still have to right-click install on each one to have it error out so you'd know it's not a Linux game. Since I already have the Linux versions of the games downloaded I'll just have to track when I downloaded a game on the Windows client and remember to do the same on the Linux client otherwise there would be alot of right-click -> install going on.....
I doubt very many people backup their games as well as crack them so likely no one cares but you'll have to do the above if you do.
The above is yet more proof (not that it's needed) that Linux native games are a joke and will forever continue to be so. Long live Windows games.
//Looks like the above console commands don't work for Linux, it still downloads Windows games. I'm not going to manually attempt to look up what games are Linux native so looks like no more downloading and backing up Linux games for me for steam. Good riddance.