Create customer_account_login.xml layout file in to your theme or extension.
Add below code in created file.
<meta name="description" content="Add description here"/>
The synthetic VCL you're showing doesn't make a lot of sense to me. I would remove the return (synth(801, "BOT")); code.
However, I would adjust the behavior, rather than falling back on the built-in VCL.
One option would be to bypass the cache when the /customer/ajax/login endpoint is reached. You probably don't want to cache this page, and ...
After the Magento upgrade, if your Magento project faces an error on the user login page, just go to the back-end of Magento(admin)>customers>all customers>(your user). Select the user and make changes from pending to approved