https://build.fhir.org/ig/HL7/fhir-subscription-backport-ig/notifications.html#fhir-r4b-and-later says :

> this guide defines the backport-related-query extension to represent the query and coded information... 

> For examples, please see Backported SubscriptionTopic: R4B Encounter Complete, R4B Notification: Id Only with Query, or R4B Notification: Full Resource with Query.
 

The first link to "Backported SubscriptionTopic" doesn't actually include a backport-related-query. I think the fix is to update the example to add one.