Can confirm, same behavior on my device. Started about 1-2 weeks ago, cant pinpoint the exact point in time nor a specific update to the system or apps. But the ability to issue voice commands via the microphone button or any hardware button results in the error message "Voice commands aren't available right now".
Interestingly, invoking the Assistant App and issuing commands there does work. But that isn't feasible in a moving vehicle, of course.