Custom Entity appears as read-only in the Unified Interface

I came across an issue where a custom entity appears read-only in the unified interface but is read-write in the legacy interface.

Soon discovered this article https://docs.microsoft.com/en-us/dynamics365/sales-enterprise/ts-unified-interface which described my issue.

In summary, the Unified Interface is using a setting on an entity previously named Read-only in Mobile.

At the time of writing the Microsoft article above showed the image referencing read-only in mobile whereas in my online instance the wording has been changed to reference the unified interface.