get timestamp of most recent comment of an article
As @IvanJaros pointed out the comments statistics are stored in a database table. You don't need an API function to get them. The comment module does this for you and places them in the field while loading the node from storage:

function comment_entity_storage_load($entities, $entity_type) {
  ...
  // Load comment information from the database and update the entity's
  // comment statistics properties, which are defined on each CommentItem field.
  $result = \Drupal::service('comment.statistics')-&gt;read($entities, $entity_type);
  foreach ($result as $record) {
    // Skip fields that entity does not have.
    if (!$entities[$record-&gt;entity_id]-&gt;hasField($record-&gt;field_name)) {
      continue;
    }
    $comment_statistics = $entities[$record-&gt;entity_id]-&gt;get($record-&gt;field_name);
    $comment_statistics-&gt;cid = $record-&gt;cid;
    $comment_statistics-&gt;last_comment_timestamp = $record-&gt;last_comment_timestamp;
    $comment_statistics-&gt;last_comment_name = $record-&gt;last_comment_name;
    $comment_statistics-&gt;last_comment_uid = $record-&gt;last_comment_uid;
    $comment_statistics-&gt;comment_count = $record-&gt;comment_count;
  }
}


So you can access the statistics directly from the node field:

$node-&gt;comment-&gt;last_comment_timestamp;
$node-&gt;comment-&gt;last_comment_name;
$node-&gt;comment-&gt;last_comment_uid;
$node-&gt;comment-&gt;comment_count;