Establish whether a node is NEW in a custom #submit function
You could try this method:

if ($form_state-&gt;getFormObject()-&gt;getOperation() == 'edit') {
  // existing node 
}


These are the operations defined for node:

 "form" = {
   "default" = "Drupal\node\NodeForm",
   "delete" = "Drupal\node\Form\NodeDeleteForm",
   "edit" = "Drupal\node\NodeForm"
 },