http://wikiba.se/ontology-1.0.owl file should be available (and serving content) so that the RDF output of Wikibase instance (Wikidata included) can be properly resolved/validated.

Given wikiba.se page/domain is operated by a third-party provider, and the owl file is not really part of wikiba.se website, but rather an additional file served from that domain, WMDE would like to ensure that file is reachable, also when the site operators are doing changes in the CMS etc.