With the news that Stadia was shutting down, Google released a tool to update the controller firmware to use Bluetooth. Someone else reverse engineered the tool to allow dumping and flashing other firmware (only the WiFi and Bluetooth firmwares exist right now though). My roommate got a Stadia controller and I decided to dump the firmware. It took 19 and a half hours to dump 16MB, see the blogpost linked above for more info on that.

# python stadiatool.py info
Found: 18d1:9400 (Google Inc. Stadia Controller)
Controller serial number: 9A220YCAC62B5B
Controller is a pvt device
Current firmware is gotham build 169429
Current battery level: 100%