The text says:

Each DocumentReference must have: ... 4. date and time the reference was created (STU3 requirement, removed in R4)

I'm not sure what to make of that... is it mandatory or not? The diff/snapshot says it is 0..1, indicating it is not mandatory – but it's listed in the "must have" section of the text above.

To further muddy the water, the "Summary" tab of the table says:

*SHOULD have: ... 2. One dateTime value in DocumentReference.created *

BUT... DocumentReference doesn't have an attribute called created. I expect it should be DocumentRefere.date.