Takashi Hatamoto investigated Salesforce's new Restriction Rules (制限ルール) feature in Summer '21. Restriction Rules allow defining which users can see which records for custom objects, contracts, tasks, and events. They provide more fine-grained access control than organization-wide sharing defaults or manual sharing. Restriction Rules are defined using the Tooling or Metadata API and allow filtering records and users based on field values. Currently, Restriction Rules are only available in Sandbox orgs due to being in beta.