Currently, the Wikibase REST API is enabled by adding $wgRestAPIAdditionalRouteFiles[] = 'extensions/Wikibase/repo/rest-api/routes.json'; to the LocalSettings.php file. This was sensible while the REST API was in beta, as it let individual instances select if they wanted to enable the REST API or not.
With the REST API v1 soon to be released, we should move the route definitions from repo/rest-api/routes.json into the extension-repo.json file so that the REST API is enabled when the Wikibase extension is loaded. This will help ensure that there is consistent functionality across all Wikibase instances.
We should also consider backporting this change to the REL1_43 LTS release.