[Update: We have new information see here.]
In a move that's not exactly surprising, Valve has quietly removed the Steam Machine section from Steam.
Previously on Steam, if you hovered over the Hardware category there was a Steam Machines link in the drop-down, which is now gone while the links to the Steam Controller, Steam Link and Vive remain. In fact, the entire Hardware page on Steam is now gone and anyone using the link (http://store.steampowered.com/hardware/) is redirected to a basic search page. Looking back on it and doing a bit of quick research, it seems the change came this month.
I'm not surprised they did this, since currently no one is announcing new machines and the whole Steam Machine idea from Valve never really gained any steam. While it didn't really do the big splash many were hoping, it has done quite a lot of good for Linux gaming overall. As a result of the initial push from Valve, many developers and game engines have moved into doing regular Linux support. This is important, because many of the barriers involved in getting games on Linux have been removed.
We know for a fact that porting companies like Aspyr Media (original interview) and Feral Interactive (original interview) started doing Linux versions thanks to SteamOS and Steam Machines, with them both still continuing the effort. It's also likely what pushed GOG to support Linux on their store too, since they didn't want to miss out on the possibility of more Linux gamers to buy games.
Realistically and looking back on it all, the time just wasn't right. There were long delays, not enough "big" games to make people truly interested in the platform (especially when the likes of The Witcher 3 was confirmed and then never happened—still hurts) and various other reasons.
We now have over four thousand Linux games on Steam, with more releasing every day. Of course, that's just a number and there's a fair amount of rubbish, but that's only natural to see. The good news, is that we get a lot of decent games arrive on Linux too and there's no signs of it slowing down.
It will be interesting to see if Valve do another SteamOS/Steam Machine push, with SteamOS still seeing updates this year it's entirely possible. Either way, Valve has done a lot of good and continues to do so. They're not a perfect company, not all their ideas work out and that's fine.
Linux gaming still faces an uphill battle—a large one at that. Thankfully, no one company "owns" Linux and so it can essentially go on forever, keep improving and gradually get better over a long time. We will still be here no matter what, we love Linux and we have so many good games already we don't know what to play half the time.
Thanks for the tip, kreativt. Article text was updated to be clearer after publishing.