Valve quietly announced that once physical stock of the Steam Link streaming device sells out, they will no longer sell physical units. Instead, Valve will shift focus and support to the Steam Link mobile app, currently only available for Android. While the reasons for the discontinuation aren't entirely clear, it's likely that the Steam Link is no longer selling enough to warrant continued support.
https://www.notebookcheck.net/Valve-to-discontinue-Steam-Link-once-current-supply-sells-out.365906.0.html