Set a Default Taxonomy for CPTs

default-tax.php

// Set "Upcoming" as Default Tax for Events (this applies on publish)
// Source: https://gist.github.com/mayeenulislam/f208b4fd408fd4742c06

function ni_set_default_object_terms( $post_id, $post ) {
  if ( 'publish' === $post->post_status ) {
    $defaults = array(
      'post_tag' => array( 'event' ),
      'event-status' => array( 'upcoming' ),
      );
    $taxonomies = get_object_taxonomies( $post->post_type );
    foreach ( (array) $taxonomies as $taxonomy ) {
      $terms = wp_get_post_terms( $post_id, $taxonomy );
      if ( empty( $terms ) && array_key_exists( $taxonomy, $defaults ) ) {
        wp_set_object_terms( $post_id, $defaults[$taxonomy], $taxonomy );
      }
    }
  }
}
add_action( 'save_post', 'bni_set_default_object_terms', 100, 2 );