Attach #after_build to field in form_alter.

module-name.module.php

<?php
  function hook_form_alter(&$form, &$form_state, $form_id) {
    if ($form_id === 'expected_form_name') { // Assuming content type containing list field.
      if (isset($form['field_settings_list'])) {
         $settingsForm = &$form['field_settings_list'];
         $settingsLang = $settingsForm['#language'];
         $settingsForm[$settingsLang]['#after_build'][] = 'field_settings_list_after_build';
      }
    }
  }
  
  // Render array for list field children is now populated.
  function field_settings_list_after_build(&$form, $form_state) {
    $lang = $form['#language'];
    $settings = $form_state['values']['field_settings_list'][$lang];
    $form['feedbackRequired']['#attributes']['disabled'] = 'disabled';
    return $form;
  }