You can now set external access levels for the Lead object, which was previously in beta, and the Campaign object. Select more restrictive access for external users without changing the default internal access level. The objects available for external org-wide defaults vary depending on your Salesforce org’s licenses and other settings.
Where: This change applies to Lightning Experience and Salesforce Classic in Professional, Enterprise, Performance, Unlimited, and Developer editions.
How: To set 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.