Valve has announced that it will be discontinuing the Steam Link hardware box. This small device allowed streaming of PC games to any display in your house via your home network. Since it was introduced in 2015, new features such as couch co-op and even 5.1 surround sound were added, but the device is sort of redundant now.
The biggest reason for the phasing out is likely the Steam Link app that is available on Android OS and some Samsung smart displays. Those options allow for 4K streaming, which is just not possible on the Steam Link hardware. Those apps are also available for free, where the dedicated hardware runs $50 (though is routinely available from Valve for $2.50).
If you'd still like to get the box, know that supply has almost dried up. The Steam Link is completely sold out in Europe and is quickly approaching that status in the US. Moving forward, Valve has stated that it, "intends to continue supporting the existing Steam Link hardware as well as distribution of the software versions of Steam Link, available for many leading smartphones, tablets, and televisions."