Call to undefined function user_load() using Drupal 9 and api3?
The function user_load is deprecated in D9.
https://api.drupal.org/api/drupal/core%21modules%21user%21user.module/function/user_load/8.7.x
Looks like we need a replacement stub function in the drupal module. For a quick hack, you could append this to /var/www/drupal/web/modules/contrib/civicrm/civicrm.module (or wherever it lives on your server).
A better method might be to create a new D9 file of functions that gets loaded selectively only for D9.
/* for when user_load gets deprecated */

if (!function_exists('user_load')) {

function user_load($uid, $reset = FALSE) {
  if ($reset) {
    \Drupal::entityManager()
      -&gt;getStorage('user')
      -&gt;resetCache([
      $uid,
    ]);
  }
  return \Drupal\user\Entity\User::load($uid);
}

}