Model/Config/ConfigRewrite.php
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;
            }
        }
    }
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;
            }
        }
    }
Set flag for purging if Fastly is switched off
@param \Magento\Config\Model\Config $subject