Enable External Org-Wide Defaults in Orgs with Communities or Portals (Critical Update, Enforced)
Where: This change applies to communities accessed through Lightning Experience and Salesforce Classic in Essentials, Enterprise, Performance, Unlimited, and Developer editions.
When: This critical update is enforced for the Summer ’19 release. Orgs that turn on communities after the Summer ’19 release is live have these defaults enabled.
How: When external org-wide defaults are first enabled, the external access levels are set to match the internal access levels. To set stricter access levels for external users, from Setup, enter Sharing Settings in the Quick Find box, then select Sharing Settings. Under Organization-Wide Defaults, edit the default external access.