In https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/566065/9/data-access/src/PerRepositoryServiceWiring.php#312 code was added to allow config switching of the backend for SQL based search (between the wb_terms table, and the new terms storage tables).
As part of the switch to EntitySources that logic has apparently disappeared and the tmpItemSearchMigrationStage and tmpPropertySearchMigrationStage options are no longer used in code, along with TermStoreDelegatingMatchingTermsLookup .
In order to allow people to actually stop using wb_terms in 1.35 if they want we need to add this functionality back into the new place in current master before the release.
When discussing in mattermost it looks like this is mainly changing https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/data-access/src/SingleEntitySourceServices.php#L301-L308 in a similar way to the code that was changed in the previous gerrit change linked in this ticket.
All of the code needed should already exist, it just needs to be wired up.