Use maxlength defined in label's entity field definition

+++ b/src/Form/ReplicateConfirmForm.php
@@ -39,6 +46,13 @@ public function buildForm(array $form, FormStateInterface $form_state, RouteMatc
 
+    $entityLabelKey = $entity->getEntityType()->getKey('label');
+    $entityFieldsDefinitions = $this->entityFieldManager->getBaseFieldDefinitions($entity->getEntityType()->id());
+    $labelMaxLength = 128;
+    if ($entityFieldsDefinitions[$entityLabelKey] instanceof BaseFieldDefinition) {
+      $labelMaxLength = $entityFieldsDefinitions[$entityLabelKey]->getSetting('max_length');
+    }

You have the entity, you can get the field definition from it. This can be simplified to $entity->getFieldDefinition($label_key). no entity field manager necessary.

The code also needs to be moved inside the existing hasKey() check, some entity types don't have a label key.

Forgot that #3106142: Allow long texts as new labels → changed the default to 255, needs to be rerolled. Could indeed cause a problem if a label is entered that's too long.