They are phasing it out but I'm going to escalate your issue to the Comcast corporate employees that are available to these boards. We've seen cases here where they have been able to get this accomplished for customers even when it is not listed in their rate cards, YMMV. You should get a reply here in your topic. Good luck !
We appreciate you for using the Xfinity Forums to get help with adding an additional IP address to your account. I'd be happy to review your account to see if this option is available to you, but would also like to let you know that the ability to have two IP Addresses on a residential account is being phased out and can only be added on Comcast Business customers.
Let's check your account to see what we can do to help. Please send me a private message. In the body of your message, please verify your first and last name and the numbers associated with your service address. To send a private message, click on "ComcastChe" and then click send a message.