class-wp-rest-posts-controller-string-terms.php

<?php

class WP_REST_Posts_Controller_StringTerms extends WP_REST_Posts_Controller {

  /**
   * Retrieves the post's schema, conforming to JSON Schema.
   *
   * @return array Item schema data.
   */
  public function get_item_schema() {
    $schema = parent::get_item_schema();

    $taxonomies = wp_list_filter( get_object_taxonomies( $this->post_type, 'objects' ), array( 'show_in_rest' => true ) );
    foreach ( $taxonomies as $taxonomy ) {
      $base = ! empty( $taxonomy->rest_base ) ? $taxonomy->rest_base : $taxonomy->name;
      if(!array_key_exists( $base, $schema['properties'] )) {
        continue;
      }
      $schema['properties'][ $base ][ 'items' ][ 'type'] = 'string';
    }

    return $schema;
  }
}