Create a taxonomy term programmatically in Drupal 8.

nodemaker_term_create.php

<?php

/**
 * @file
 * Contains various helper functions.
 */

use Drupal\taxonomy\Entity\Term;

/**
 * Helper function to create a taxonomy term programmatically.
 *
 * @code
 * // Create top level term
 * $term_id = _nodemaker_term_create('My Term', 'my_vocab', []);
 *
 * // Create term with parent term with an id of 999
 * $term_id = _nodemaker_term_create('My Term', 'my_vocab', [999]);
 * @endcode
 *
 * @param string $term
 *   - Term Name.
 * @param string $vocabulary
 *   - System id of the vocabulary term will be added to.
 * @param array $parent
 *   - Array of term ids to be assigned as parent.
 *
 * @return int|null
 *   - Returns the term id of the created term on success, null on failure.
 */
function _nodemaker_term_create($term, $vocabulary, array $parent = []) {

  // Create the taxonomy term.
  $new_term = Term::create([
    'name' => $term,
    'vid' => $vocabulary,
    'parent' => $parent,
  ]);

  // Save the taxonomy term.
  $new_term->save();

  // Return the taxonomy term id.
  return $new_term->id();
}