Model/PageCache/PurgeCachePlugin.php
public function aroundSendPurgeRequest(PurgeCache $subject, callable $proceed, ...$args) // @codingStandardsIgnoreLine - unused parameter
    {
        if ($this->config->isFastlyEnabled() !== true) {
            $proceed(...$args);
        }
    }
public function aroundSendPurgeRequest(PurgeCache $subject, callable $proceed, ...$args) // @codingStandardsIgnoreLine - unused parameter
    {
        if ($this->config->isFastlyEnabled() !== true) {
            $proceed(...$args);
        }
    }
Prevent Magento from executing purge requests on Varnish when Fastly is enabled

@param PurgeCache $subject
@param callable $proceed
@param array ...$args