Enterprise architecture (EA) is the most important thing to review in this situation, as it provides a holistic view of the current and desired state of the IT applications, data, and infrastructure, as well as the business processes, capabilities, and goals that they support. EA can help identify the redundant IT applications, the data sources and dependencies, the integration requirements and challenges, and the alignment with the strategic initiative of providing integrated services to customers. EA can also help define the roadmap, standards, and governance for achieving the desired state of IT integration.
IT risk register, balanced scorecard measures, and IT strategic plan are also important things to review, but they are not as essential as EA in this situation. IT risk register is a document that records the IT risks that may affect the enterprise's objectives and operations, as well as their likelihood, impact, mitigation strategies, and status. IT risk register can help identify and manage the potential risks associated with IT integration, such as data quality, security, compatibility, performance, and compliance issues. Balanced scorecard measures are a set of metrics that track the performance of IT in relation to the enterprise's vision, strategy, and goals. Balanced scorecard measures can help evaluate the effectiveness and efficiency of IT integration, as well as its contribution to customer satisfaction, business value, and innovation. IT strategic plan is a document that outlines the vision, mission, objectives, initiatives, and actions of IT to support the enterprise's strategy and goals. IT strategic plan can help align IT integration with the business needs and expectations, as well as allocate the necessary resources and budget for it.
References := Enterprise Architecture Governance - CIO Wiki; Enterprise Architecture Governance | The Definitive Guide | LeanIX; Enterprise Architecture Governance - Why It Is Important (Part 2); What is IT governance? A formal way to align IT & business strategy.

Upon learning that business units have been independently procuring cloud services, the IT steering committee's best course of action is to define a procurement strategy based on business unit needs. This approach ensures that cloud service procurement aligns with the enterprise's overall IT strategy and governance policies while still addressing the specific requirements of individual business units. It fosters collaboration between IT and business units, ensuring that cloud services are vetted for compliance, security, and interoperability. Requiring cancellation, including business unit leadership in the EA review board, or limiting usage to open-source solutions may address aspects of the issue but do not provide a comprehensive strategy that aligns business needs with IT governance.