A public group is a grouping of users, roles, roles and subordinates, or other groups that can be used to share access to records or folders. A sharing rule is a rule that grants additional access to records based on certain criteria or ownership. In this case, the administrator should create a public group for the renewals team and create a criteria based sharing rule on opportunities that grants read-only access to the group for all closed won opportunities. This way, the renewals team can see all the opportunities that are eligible for renewal while keeping the account private. References:
https://help.salesforce.com/s/articleView?id=sf.admin_sharing.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.sharing_rules.htm&type=5

Data Classification is the feature that the administrator should use to flag where sensitive information is stored in Salesforce. Data Classification is a feature that allows administrators to flag where sensitive information is stored in Salesforce by assigning sensitivity and classification levels to fields. Data Classification can help administrators comply with data privacy regulations and best practices by identifying and managing sensitive data in their orgs.