It looks like Valve has quietly removed the Steam Machine section from Steam entirely. Before this change if you hovered over the Hardware category there was a Steam Machines link in the drop-down, which is now gone. Links to the Steam Controller, Steam Link, and Vive remain. Actually the entire Hardware page on Steam is now gone and anyone trying to go to it is redirected a search page.
This is really no surprise, no one is really announcing Steam Machines anymore and the whole Steam Machine movement really did not gain much traction. The real win for Steam Machines was adding and doing regular Linux support for games and game engines. This removed many of the barriers in getting games in Linux.
There are over 4000 Linux games on Steam with more being added every day. It is a good thing these days that many great titles do arrive on Linux now, whereas before this just didn’t happen.