See https://api.backdropcms.org/node/26881

Example: Create a Taxonomy Term

    $term = entity_create('taxonomy_term', array(
      'name' => 'Test term',
      'vocabulary' => $vocabulary->machine_name,
      'description' => NULL,
      'format' => 1,
    ));
    taxonomy_term_save($term);

Example: Create a new Vocabulary object

$vocabulary = new TaxonomyVocabulary(array(
  'name' => 'New Vocabulary',
  'description' => 'My description',
  'machine_name' => new_vocab',
  'weight' => 5,
));
taxonomy_vocabulary_save($vocabulary);
return $vocabulary;

However this does not include the introduced enforceIsNew() method from the corresponding Drupal issue at https://drupal.org/node/1361232. Backdrop maintains the existing is_new() property for API compatibility.

Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/154
https://github.com/backdrop/backdrop-issues/issues/49