code: public function beforeSave(\Magento\Config\Model\Config $subject)
      {
          $data = $subject->getData();
          if (!empty($data['groups']['full_page_cache']['fields']['caching_application']['value'])) {
              $currentCacheConfig = $data['groups']['full_page_cache']['fields']['caching_application']['value'];
              $oldCacheConfig = $this->scopeConfig->getValue(\Magento\PageCache\Model\Config::XML_PAGECACHE_TYPE);
  
              if ($oldCacheConfig == \Fastly\Cdn\Model\Config::FASTLY && $currentCacheConfig != $oldCacheConfig) {
                  $this->purge = true;
              }
          }
      }
docs: Set flag for purging if Fastly is switched off
  @param \Magento\Config\Model\Config $subject