NEC purports to cover all kinds of substation. See below.NEC just does not cover the kind of substations where earthing would matter any.
May be the modern earth fault relay is so sensitive that suitable sensitivity may be chosen to operate at an earth fault current no matter how low it may be. But for substation with moderately sensitive earth fault relay, low ground resistance may still be required for the operation of ground fault relay.
250.191 Grounding System at Alternating-Current
Substations. For ac substations, the grounding system shall
be in accordance with Part III of Article 250.