Get value of field from a node inside region

themename.theme.php

<?php

/**
 * Implements hook_preprocess_HOOK() for region templates.
 */
function themename_preprocess_region__region_name(&$variables) {
  // Check if current page is a node.
  if ($node = \Drupal::routeMatch()->getParameter('node')) {
    // Check if node has the field site_section.
    if ($node->hasField('field_site_section')) {
      $variables['site_section'] = $node->get('field_site_section')->value;
    }
  }
}