How do I get a plaintext list of tags from a multi value entity reference field?
You can get the array of reference entities from node using 


  $node->get('field_tags')->referencedEntities();


So the code will look something like below:

// Get the referenced entities
$tags = $node-&gt;get('field_tags')-&gt;referencedEntities();
// Check if we have output. (Assuming we get an array, You will have to check what your output it.)
if (!empty($tags)) {
  // Go through each entity.
  foreach ($tags as $tag) {
    // Write your Business Logic here to create your Keywords Array.
  }
}
// Use the Keywords Array for further use.


I Hope this helps you.