Problem/Motivation
EntityDisplayBase::handleHiddenType is a method added in as a BC layer and marked deprecated despite being needed and used in Drupal 8 core.
Proposed resolution
Properly trigger an error when the BC layer is activated so contrib knows to not use type => hidden. Undeprecate the specific method as it is currently in use. When the Drupal 9 branch is opened, remove usages, empty the method, and mark as deprecated for removal in Drupal 10.
Follow up issue is at #2799641: Remove automatic 'type' handling from EntityDisplayBase in favor of explicit 'region' handling
Remaining tasks
User interface changes
API changes
handleHiddenType will be properly deprecated