Wikibase has a getLocationAgnosticMwApi function (in wikibase/javascript-api) which returns an instance of either mw.Api or mw.ForeignApi, depending on whether the target URL matches the current host or not. I think this is no longer necessary: since we resolved T208601, it should be fine to simply use ForeignApi unconditionally. We should migrate callers to that simpler pattern and then get rid of this method.