Wow, got really burned by Apple’s terrible documentation. The App Attest API exists on macOS, but DCAppAttestService.isSupported just returns false with no indication why. I thought I had misconfigured something.
The only hint that App Attest is not actually supported on macOS is that there is no App Attest capability entitlement when you click around in Xcode for macOS, only for other platforms.
#macosdev