While reviewing #1946466-58: Convert all confirm_form() in user.module and user.pages.inc to the new form interface and convert route I found.
+++ b/core/modules/user/lib/Drupal/user/Form/UserMultipleCancelConfirm.phpundefined
@@ -0,0 +1,220 @@
+ $form['accounts'] = array('#prefix' => '<ul>', '#suffix' => '</ul>', '#tree' => TRUE);
+ foreach ($accounts as $uid => $account) {
+ // Prevent user 1 from being canceled.
+ if ($uid <= 1) {
+ continue;
+ }
+ $form['accounts'][$uid] = array(
+ '#type' => 'hidden',
+ '#value' => $uid,
+ '#prefix' => '<li>',
+ '#suffix' => String::checkPlain($account->label()) . "</li>\n",
+ );Me(@jibran) and @tim.plunkett unable to understand why
'#prefix' and '#suffix' is used with '#type' => 'hidden'