Is there any guarantee about JSON object fields?

I’m working on an Asana client library for Rust. In the developer docs, specifically the schema section, is there any guarantee about which keys in each object will be present? For example, the User object contains several fields, including the photo field. Would the photo field ever be omitted because the user doesn’t have any photos? Of course that’s just an example. Is every field in the schema documentation guaranteed to be present? Is there any information about which fields would be omitted?

Thanks in advance.