This is a list of video games and demos that are available but hidden from the Steam store.
You need to insert steam://install/<Steam app ID> (Windows/OS X) or app_install <Steam app ID> in your browser to install the hidden application. It will ask you if you want to "Launch Application", that application being Steam. You can use SteamDB to find the correct Steam app id.
steam://install/<Steam app ID>
app_install <Steam app ID>
You can also do this by openening the Windows’ “run” dialogue box which can be acessed by pressing the Windows key + R.
Some of the older applications might not be compatible with Windows 10 while others might have been remove due to technical issues and/or bugs.