I do understand historical context as a precondition to historical understanding of poetry. Equally, it appears that poems must be put into historical context in order for the general public (including the bright young things at university) to grasp meaning. A friend yesterday noted that his high schooler's best introductory assignment was the selection of 10 or 20 poems upon one historical theme.
However, I would suggest that it is perhaps somewhat too narrow to assert that poems such as Brooke's cannot still be written--in other places at this very moment. Neither history or poetry are governed by immutable laws.