The element Questionnaire.experimental is optional but currently does not have a meaning-if-missing text. The change request here is to add this information, to give guidance for implementations on how to treat the resource if the element is missing (especially given that it is a modifier element). 

It would also seem like a good idea to also add this information to experimental elements in other resources, e.g. in CodeSystem, ValueSet, and StructureDefinition.

See also this Zulip thread for background: https://chat.fhir.org/#narrow/stream/179166-implementers/topic/Optional.20fields.20that.20seems.20to.20require.20default.20value.20in.20base