Use Drupal 8 block region in template suggestions

MODULENAME.module.php

<?php

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function THEMENAME_theme_suggestions_menu_alter(array &$suggestions, array $variables) {
  if (isset($variables['attributes']['region'])) {
    // Add a template suggestion based on region name
    $suggestions[] = $variables['theme_hook_original'] . '__' . $variables['attributes']['region'];
  }
}
 
/**
 * Implements hook_preprocess_block().
 */
function THEMENAME_preprocess_block(&$variables) {
  if (isset($variables['elements']['#id'])) {
    $block_id = $variables['elements']['#id'];
    $block = \Drupal\block\Entity\Block::load($block_id);
 
    if ($block) {
      // Add a region variable to a block.
      $variables['content']['#attributes']['region'] = $block->getRegion();
    }
  }
}