Note about plain text: on this page it means “not HTML” — emojis, for instance, are considered plain text.
url (optional, string) is the URL of the resource described by the item. It’s the permalink. This may be the same as the id — but should be present regardless.
The 'spec' is full of this sort of thing. "This is optional, but must be there." "This is mandatory, but you can omit it."
Oh look, another:
content_html and content_text are each optional strings — but one or both must be present.
I somehow think the author is unaware of things like RFCs. Or peer review before publishing.