The change in MDL-68690 means the add_documents function in search engine plugins is expected to return 6 values. There is backwards compatibility for 5 return values in two places (the index and index_context functions in search/classes/manager.php) which should be deleted in Moodle 4.2.

Note: I have currently added this as a subtask of my main item, it should be moved to a sub-task of the epic for deprecations in 4.2, but this doesn't exist yet.