According to this page (https://docs.backdropcms.org/documentation/features-added-to-core), Views Bulk Operations has been added to core.
Was this a partial integration or has all the functionality of the Drupal 7 version of the module been integrated into core?
Not all the functionality has been integrated. Some missing features are listed in the meta issue https://github.com/backdrop/backdrop-issues/issues/3797 .