undefined is a Javascript construct, not a JSON one. While I recognize that undefined would be useful in the situation mentioned in the task description, we want REST responses to conform to valid JSON schemas.

As for completely omitting values, that does occur in some situations (you won't get an newer value from /page/{title}/history if you're on the first page of results, but you will on the other pages.

In the cases where null is returned, as opposed to omitting the value, that was generally an intentional decision for the contract of that particular response. I don't have all those original discussions on hand. But if there are specific cases where you'd like that reconsidered, feel free to post a more specific task (or one task with a bunch of them would be a fine starting place too).