It looks to me like the prepositional phrase is "of our shift explained the new policy."
Explanation:
A prepositional phrase is a part of a sentence that consists of one preposition and the object it affects. The word "of" is a preposition, therefore, the phrase is "of our shift explained the new policy."
In June 2015, the TRC released an Executive Summary of its findings along with 94 "calls to action" regarding reconciliation between Canadians and Indigenous peoples.
