In a post on the Steam Community website, Valve writes, “The supply of physical Steam Link hardware devices is sold out in Europe and almost sold out in the US. Moving forward, Valve intends to continue supporting the existing Steam Link hardware as well as distribution of the software versions of Steam Link, available for many leading smart phones, tablets and televisions.”
For those who are unfamiliar with the Steam Link, this was a device that allowed gamers to stream their games from their PCs onto another device, like a TV. This meant that if you had the necessary input devices hooked up to your TV, in theory you could use the Steam Link to stream games onto your TV that would otherwise require a PC to play.
It is unclear why Valve has discontinued it given its usefulness, but the company has since expanded the idea of Steam Link onto mobile devices, where they launched the Steam Link app on Android which achieved a similar outcome. The app was also initially intended to arrive for iOS devices too, but Valve has been facing some resistance from Apple due to alleged guideline violations.