Once T214557 is finished (functionality is written and complete) and useEntitySourceBasedFederation is true everywhere in production and by default in Wikibase we can remove the old code that it is switching away from.
[x] {T245547}
[x] Remove all conditional code paths in Repo and Client that were differing depending on the value of `useEntitySourceBasedFederation` setting
[x] {T245817}
[x] {T245830}
[x] {T245831}
[x] {T245860}
[x] Remove `ForeignEntityValidator`: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/574018
[x] {T245865}
[x] Remove `UnusableEntitySource`: https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Wikibase/+/574192
[x] {T245870}
[x] Repo: Rename `WikibaseRepo::getEntityTypeToRepositoryMapping` - given "repository" concept will be gone
[] Remove the logic setting up "entity source" config from "legacy" setting options (`repositories`, `foreignRepositories`, `conceptBaseUri`, `entityNamespaces`, ...)
[x] Use the new settings (entitysources) on WMF wikis: T254315
[] Clean up any remainders of the legacy back/compat config in Wikibase and extensions: T257262
[] Remove the legacy setting options from the configuration of WMF wikis: T257260
[x] Update services documentation https://gerrit.wikimedia.org/r/c/577313
~~(possibly optional (please talk to the federated properties team folks before doing this)) Remove `EntityId::getRepositoryName` and "repository prefix" related code in Wikibase Data Model library~~