How to override text field widget form in Drupal 8 In my project I am using maxlength module to display the message like 
'Content limited to 100 characters, remaining: 20'. The module using the hook 'hook_field_widget_third_party_settings_form' and provide textboxes to add length size and count down message.

I want to alter this count down message for all the fields by using my own custom module. Please help on this ?
You can create you own widget for textfield you will use in your forms each time you need :

&lt;?php

namespace Drupal\your_module\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\Plugin\Field\FieldWidget\StringTextareaWidget;
use Drupal\Core\Form\FormStateInterface;

/**
 * Plugin implementation of the 'your_widget_widget' widget.
 *
 * @FieldWidget(
 *   id = "your_module",
 *   label = @Translation("Your widget name"),
 *   field_types = {
 *     "string"
 *   }
 * )
 */
class YourWidget extends StringTextfieldWidget {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &amp;$form, FormStateInterface $form_state) {
    $element['value'] = $element + [
      '#type' =&gt; 'textfield',
      '#default_value' =&gt; $items[$delta]-&gt;value,
      '#placeholder' =&gt; $this-&gt;getSetting('placeholder'),
      '#maxlength' =&gt; $this-&gt;getFieldSetting('max_length'),
      '#attributes' =&gt; [
        'class' =&gt; ['js-text-full', 'text-full'],
      ],
    ];

    return $element;
  }

}


You probably have to override settingsForm() with your maxlength settings. 
You can do this for each widget type (textarea...)

Note that you can generate skeleton with this drupal command :

$ drupal generate:plugin:fieldwidget
$ drupal gpfw