diff -u b/src/WebformSubmissionStorage.php b/src/WebformSubmissionStorage.php
--- b/src/WebformSubmissionStorage.php
+++ b/src/WebformSubmissionStorage.php
@@ -57,16 +57,6 @@
   protected $accessRulesManager;
 
   /**
-   * The entity field manager.
-   *
-   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
-   *
-   * @todo Remove past drupal:8.6.x support, as from drupal:8.7 on, this is
-   *   supplied by ContentEntityStorageBase.
-   */
-  protected $entityFieldManager;
-
-  /**
    * The entity type manager.
    *
    * @var \Drupal\Core\Entity\EntityTypeManagerInterface
@@ -89,7 +79,6 @@
     $this->currentUser = $current_user;
     $this->accessRulesManager = $access_rules_manager;
     $this->time = $time ?: \Drupal::time();
-    $this->entityFieldManager = \Drupal::service('entity_field.manager');
     $this->entityTypeManager = \Drupal::service('entity_type.manager');
   }
 
@@ -113,8 +102,9 @@
    * {@inheritdoc}
    */
   public function getFieldDefinitions() {
-    /** @var \Drupal\Core\Field\BaseFieldDefinition[] $definitions */
-    $field_definitions = $this->entityFieldManager->getBaseFieldDefinitions('webform_submission');
+    // @todo: Replace by entityFieldManager's getActiveFieldStorageDefinitions()
+    // once drupal:8.6.x is not supported anymore.
+    $field_definitions = $this->getFieldStorageDefinitions();
 
     // For now never let any see or export the serialize YAML data field.
     unset($field_definitions['data']);
@@ -360,8 +350,8 @@
     $options = [];
     $source_entities = $this->getSourceEntities($webform);
     foreach ($source_entities as $entity_type => $entity_ids) {
-      $optgroup = (string) $this->entityFieldManager->getDefinition($entity_type)->getCollectionLabel();
-      $entities = $this->entityFieldManager->getStorage($entity_type)->loadMultiple($entity_ids);
+      $optgroup = (string) $this->entityTypeManager->getDefinition($entity_type)->getCollectionLabel();
+      $entities = $this->entityTypeManager->getStorage($entity_type)->loadMultiple($entity_ids);
       foreach ($entities as $entity_id => $entity) {
         if ($entity instanceof TranslatableInterface && $entity->hasTranslation($this->languageManager->getCurrentLanguage()->getId())) {
           $entity = $entity->getTranslation($this->languageManager->getCurrentLanguage()->getId());