Hi, Is there standard groove depth used for aircraft tires
I would not assume that there is a standard for new tires, the standard is probably set by the manufacturer and wear based on composition of material.
As for a tire being unservicable, it is usually common to remove on spot of the indicator ply. This ply has no other function but to warn of being close to an actual functional layer of the tire being close to damage. I have seen 3 layers of indicator ply exposed (on retreads), they reapply the indicator tread over the old often.
May not be your interest but looking at a tire that was drug flat shows the many (structural chords) that holds a tire together. A bald tire is nothing more than a tire that has a bit less traction on a wet runway. Not a major concern.