Notes:

• Split this into two tasks about sitelinks and external ID
• AC would include having a new query field and response to be a single item or none
• For sitelink the field name would be "itembysitelink" and would take the site id and the title OR just the site id as input
• For external ID the field name would be "itembyexternalD" and would take the external id and its corresponding property id as input

Can be tested on https://test.wikidata.org/w/api.php?action=wbgraphql&format=json e.g. with the following query:

{
  itemBySitelink(siteId: "enwiki", title: "Test") {
    id
  }
}

Change #1266856 had a related patch set uploaded (by Jakob; author: Jakob):

[mediawiki/extensions/Wikibase@master] GQL: Fix itemBySitelink when no item is found

https://gerrit.wikimedia.org/r/1266856

Change #1266856 merged by jenkins-bot:

[mediawiki/extensions/Wikibase@master] GQL: Fix itemBySitelink when no item is found

https://gerrit.wikimedia.org/r/1266856

The bug where the API errored when there is no item for the requested sitelink is fixed now. Can be tried out here: https://doc.wikimedia.org/Wikibase/master/js/graphql-explorer/?query=ewogIGl0ZW1CeVNpdGVsaW5rKHNpdGVJZDogImVud2lraSIsIHRpdGxlOiAiVGhpcyBkb2VzIG5vdCBleGlzdCIpIHsKICAgIGlkCiAgfQp9

confirmed, thank you!