Initially T305882 suggested to use HTTP 301 response code to indicate requested item has been merged/redirected to another item.

While it might not make much difference for GET requests, per the overall industry's recommendation, e.g. https://tools.ietf.org/id/draft-hunt-http-rest-redirect-00.html, 308 response code should be preferred. It would the response code to use for e.g. POST requests in Wikibase REST API, and for overall consistency it makes sense that GET requests result in similar responses.

Therefore, unlikely originally suggested, GET /entities/items/{item_id} should result in 308 HTTP response when request item has been redirected.