Use Picsum.photos to generate some random random avatars.

sul-demo-avatars.php

<?php
/**
 * Plugin Name:       Simple User Listing - Demo Avatars
 * Description:       Use Picsum.photos to generate some random random avatars.
 * Author:            helgatheviking
 * Version:           1.0.0
 * Requires at least: 6.5
 */

defined( 'ABSPATH' ) || exit;

function simple_user_listing_random_avatars($args) {
  $args['url'] = 'https://picsum.photos/120?random='. uniqid();
  return $args;
}
add_filter('pre_get_avatar_data', 'simple_user_listing_random_avatars');