Valve does not mention Steam Machines any longer on Steam. If you check it out, the links have been removed from Steam, and if you try to load up an old page on it, you get redirected. Steam Machines never became a viable commercial success.
Earlier in March Valve seems to have removed the Steam store page for Steam Machines, prebuilt computers running SteamOS, a Linux distribution built for gaming Valve has not announced anything on this neither do the disclose any info, however website GamingOnLinux discovered this.
This removal seems to be the end of Steam Machines, which failed to ever really be relevant and often underpowered. The good thing that came out of it, however, is SteamOS, a product that Valve is continuing to develop today, and will likely continue to get support in the future.
-- GOL --
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 and Feral Interactive 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.