Get menu link item from MenuLinkTreeElement
You can get the menu link content entity by accessing the uuid with the method getDerivativeId() of the plugin:

if ($link instanceof \Drupal\menu_link_content\Plugin\Menu\MenuLinkContent) {
  $uuid = $link-&gt;getDerivativeId();
  $entity = \Drupal::service('entity.repository')
    -&gt;loadEntityByUuid('menu_link_content', $uuid);
  $field_value = $entity-&gt;field_example-&gt;value;
}