I think something broke in GCController code on macOS. I downloaded the "Fox2" sample code from WWDC21, connected my DS4 controller via bluetooth, and ran the project; it registers the controller is connected and even sends haptic feedback to the controller when I use keyboard inputs, but doesn't get inputs back from the controller.
[Update] I just updated to 15.2 and it's working again. Maybe I should have just tried restarting the Mac first.