I'm making progress to wrap my mind around the thought process given in designing the metadata model. Traditionally, a "Read-only" thing means it is "present" to begin with. Consider this statement from the link: https://linkedin.github.io/rest.li/Validation-in-Rest_li#restli-validation-annotations "For example, 

@ReadOnly

 should only be used to enforce that an optional field is not present. It should not be specified for a required field, making missing required field value valid." It felt awkward. In fact I didn't understand these 2 sentences. It's hard for me to align my general understanding with this seemingly new way to describe ReadOnly and CreateOnly validations.

Do note that DataHub doesn't really use these Rest.li annotations.