Access field value in template_preprocess_views_view_field function?
It depends how you have your view set up.

If you have your view using the format 'Fields' then you can override the content with template_preprocess_views_view_field() like you say, like this (although you might want to target individual fields with some if statements):

/**
 * Implements hook_preprocess_HOOK() for views-view-field.html.twig.
 */
function THEMENAME_preprocess_views_view_field(&amp;$variables) {
  $view = $variables['view'];
  $field = $variables['field'];

  if ($view-&gt;id() == 'VIEW_NAME' &amp;&amp; $view-&gt;current_display == 'DISPLAY_NAME') {
    if ($field-&gt;field == 'FIELD_NAME') {
      // Replace characters in the field output.
      $variables['output'] = Drupal\Core\Render\Markup::create(str_replace('find', 'replace', $variables['output']));
    }
  }
}


If you are using the format 'Content' then you could do it at the field template level or node template level instead, since in that case it doesn't use the views field templates.