Even if the official tool does shut down, a quick search shows several projects on GitHub to replace the official flashing utility. This seems to include a backup of the original site [1], an extended version [2] and a dump of the firmware [3].
[1] https://github.com/bestadamdagoat/stadiaupdatetool
[2] https://github.com/luigimannoni/stadia-controller-flasher