I made an AUR version of SteamLink for RPi.
https://aur.archlinux.org/packages/steamlink-raspberrypi/
It is a repackage of the official .deb installer, which downloads and installs the actual SteamLink client when run for the first time as an user. It should work on Rpi3 and 4, unfortunately on armv7h only, because the SteamLink client is 32-bit.
It is still a bit of a work-in-progress, so please test and give feedback. And if you are using pulseaudio and the sound is all garbled while using SteamLink, here is the fix: https://steamcommunity.com/app/353380/discussions/6/1642042464753800526/