Google has put up a web-based tool to turn on the Bluetooth mode for the Stadia Controller, as we finally say goodbye.
They say it works on Chrome 108 and above but when trying to work it on Fedora KDE, it just gives a message about currently being in use by another tab or program no matter what I tried. Even setting up the special udev rules they note in the FAQ, just nothing seemed to work. I had to dust off my old Windows install to do it, where it worked without problems there.
The switch is permanent, and headphones won't work when plugged into the bottom of the controller when using Bluetooth mode. Tandem Mode will still work though, where you connect another controller to it but they both act like a single controller.
Once you've managed to enable Bluetooth, you can hold down Stadia + Y to start pairing and continue using it. I'm quite thankful it hasn't ended up as e-waste, because it's damn comfortable to hold. Easily still one of the nicest ever feeling controllers I've used.
Testing it briefly with my Steam Deck in the new Bluetooth mode — it works just fine connecting, and a quick game test showed it fine too.
Plus, the biggest and most important thing to note: there's a time limit. You only have until 31st December 2023 to enable it. Quite a while and no doubt because they just want to shut down everything consumer-side for it and be finally rid of it.
See more on their FAQ.