Hello @craftyguy, please go to Steam -> Settings -> View Account Details -> Preferences and scroll down to Platform Preferences. Please verify that that your OS preferences are set to what you want in that section.
@kisak-valve Yep, it's set to only select Linux/SteamOS:

But the store still displays a lot of titles that don't work on this OS:

I would like to second that i have this issue, even when non-Linux games are excluded by default from elsewhere on the website (as per my preferences).
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
Algorithms in the steam client should filter out recommendations for products that won't work on the OS where the steam client is installed/running. Steam clearly knows when it's running on Linux, for example, because the Steam survey and various warnings show up when trying to run a non-native game.
Steps for reproducing this issue: