Final answer:
The underlined verb phrase 'had been isolated' is in the Progressive Perfect aspect, indicative of a past ongoing action that was completed.
Step-by-step explanation:
The underlined verb phrase 'had been isolated' is in the Progressive Perfect aspect. This aspect is used to describe actions that were ongoing over a period of time in the past and were completed at some point in the past. In the sentence 'The shipwrecked sailors had been isolated for decades', it shows that the sailors were continuously isolated over the course of decades and that this state of isolation had ended by the time of speaking or referencing.