Expeditionary Diplomacy Act of 2021 This bill modifies oversight and accountability procedures for diplomatic personnel and actions by revising the structure and scope of review for certain diplomatic activities. Specifically, the bill requires the Department of State to provide quarterly briefings (currently held monthly) on progress towards opening or reopening high-risk, high-threat posts, as well as risks to national security from their continued closure and any barriers to opening those posts. The bill also recasts and revises the responsibilities of Accountability Review Boards (i.e., the mechanism through which the State Department currently investigates any event that causes serious injury, loss of life, or significant destruction of property at, or related to, a U.S. government mission abroad). The bill renames these boards Security Review Committees, and it requires these committees to take into account specified contextual and mitigating factors when evaluating diplomatic actions that led to such outcomes, as well as the diplomatic value of operations relating to the incident in question.