Get Field Name Being Deleted From Configuration Updates/Delete Form
Try with hook_form_alter something like 

/**
 * Implements hook_form_alter().
 */
function MyModule_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == "field_config_delete_form"){
    $field = $form_state-&gt;getFormObject()-&gt;getEntity();
    \Drupal::messenger()-&gt;addWarning("You are going to delete field : " . $field-&gt;getName());
  }
 }