It looks like Valve is finally ready to let go of its Steam Machines project, as the tab has now been removed entirely from Steam. It appears the whole concept has been abandoned, with no fanfare, which is actually quite disappointing. Previously, if you went to the Steam Store page and went to peruse the Hardware tab, you'd be greeted with Steam Machines in addition to the Steam Link, Steam Controller, and HTC Vive. Now, Steam Machines is all gone, and you can't search for any of them in the section.
In fact, you can't even see Steam Machine configurations or their pricing structures on the page, which was noted by GamingOnLinux. It isn't exactly surprising that the products have been swept under the rug, seemingly overnight, however. The program was first announced in 2013 when SteamOS was revealed, and the PCs simply never really took off. Multiple problems, such as a lack of exclusives, the pricey hardware overall, and even the Steam Link, made it difficult for consumers to really buy in to the products.
If you want to check out what could have been, but what was apparently never destined to be, you can visit the Steam Machines page on Steam as it still exists via direct link, but don't expect any references or any further support for the systems going forward. It looks like this is the end of an era for a product that never truly got off the ground to begin with.
Luckily, building a gaming PC is a process that's only gotten easier with time, so if you're looking to go that route at some point in the future, you still have the option to do so. But you won't have Steam Machines as a viable (but pricey) option.