In #3238635: Move AutocompleteBackendInterface to Search API, the AutocompleteBackendInterface interface was added to the Search API module itself, so that other modules (like the DB, Solr, ES, OS backend modules) can use it without adding a dependency on Search API Autocomplete.
Consequently, the AutocompleteBackendInterface interface inside Search API Autocomplete (\Drupal\search_api_autocomplete\AutocompleteBackendInterface) has been deprecated and should be replaced by the new one (\Drupal\search_api\Contrib\AutocompleteBackendInterface).
Additionally, it is suggested that backend plugins supporting the search_api_autocomplete feature should start implementing this interface as soon as they can rely on the corresponding version of the Search API module (8.x-1.22 or higher).
