To better secure your data, the External Sharing Model is enabled by default in all Salesforce orgs created in Spring ’20 or later. External org-wide defaults let you set more restrictive levels of access for external users, instead of giving internal and external users the same default access. In these newly created orgs, external access levels are initially set to Private for all objects.
Where: This change applies to Lightning Experience and Salesforce Classic in Professional, Enterprise, Performance, Unlimited, and Developer editions.
How: We recommend that you maintain a strict access level for external users, but you can edit your external org-wide defaults. From Setup, enter Sharing Settings in the Quick Find box, then select Sharing Settings. Under Organization-Wide Defaults, edit the default external access.