Problem/Motivation
On #2900409: [Meta] Improve UI of Reference field settings form, Berdir noticed that the code for that patch was very similar to the views_entity_field_label() function.
Rather than maintaining this code in two places, it would be better if it was moved to be a method on a class accessible as a service. Berdir suggested maybe putting on the EntityFieldManager class.
Proposed resolution
Move to EntityFieldManager
Make views_entity_field_label() call that method.
Deprecate views_entity_field_label() -- meaning adding @deprecated tag to the doc block, adding trigger_error to the code, and replacing calls to that function in core.
Remaining tasks
Follow up:
Once that is done, we can also use the new function on #2900409: [Meta] Improve UI of Reference field settings form (which will be taken care of on that issue, not here).
User interface changes
None.
API changes
function views_entity_field_label() will be deprecated in favor of \Drupal::service('entity_field.manager')->getFieldLabels($entity_type_id, $field_name);
Data model changes
None.