Allows you to use `rand` as an `orderby` parameter when querying a custom post type through the WordPress REST API

add-rand-orderby-to-rest.php

<?php
class Add_Rand_to_OrderBy {
  public function __construct() {
    add_action( 'rest_api_init', array( $this, 'rest_api_init' ) );
  }
  
  /**
	 * Set up any custom REST API handlers
     *
     * @access public
     * @since  2.0
     * @return void
	 */
	public function rest_api_init() {
    $post_type_slugs = array( 'sponsor' );
    foreach ( $post_type_slugs as $s ) {
  		add_filter( "rest_{$s}_collection_params", array( $this, 'rest_orderby_rand' ), 10, 1 );
    }

	/**
	 * Add rand as an orderby parameter for sponsor REST API queries
     * @param array $params the array of parameters
     *
     * @access public
     * @since  2.0
     * @return array the updated array of parameters
	 */
	public function rest_orderby_rand( $params ) {
	    $params['orderby']['enum'][] = 'rand';
	    return $params;
	}
}