Yes, officially this is a documentation bug.  From [the *Absent Fields* doc][1]:

> Fields that are not always guaranteed to be set... are indicated in the per-type documentation. **Any discrepency(sic) between observed behavior and the documentation should be reported on Stack Apps.**

(Emphasis added and note that "discrepancy" is mispellerized. ;) )

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`notice` obviously should have the `may be absent` flag.
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I eyeballed all of the other properties of [type question][2], as well as [type answer][3] and [type user][4].  I didn't spot any other obvious discrepancies with the `may be absent` flag.


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Re: 

> The notice type, that is a field of a question, appears to be optional, yet it is not marked as "may be absent"?

Optional and `may be absent` are separate dimensions.

 - "Optional" -- means you must craft/use a custom filter to get the property.
 - "May be absent" -- means that even if called for by the filter, the property will still not be returned if the property's value is null for that particular question.

  [1]: https://api.stackexchange.com/docs/absent-fields
  [2]: https://api.stackexchange.com/docs/types/question
  [3]: https://api.stackexchange.com/docs/types/answer
  [4]: https://api.stackexchange.com/docs/types/user