To better protect your Salesforce org’s data, you can no longer disable the external sharing model after it’s enabled in your org.
Where: This change applies to Lightning Experience and Salesforce Classic in Professional, Enterprise, Performance, Unlimited, and Developer editions.
How: The external sharing model allows you to customize the access levels for internal and external users. We recommend setting org-wide defaults to Private for external users.
From Setup, enter Sharing Settings in the Quick Find box, then select Sharing Settings. Under Organization-Wide Defaults, edit the Default Internal and External Access as required by your business needs.