No new unlimited data plans are available, as they are no longer offered. If you are referring to a grandfathered unlimited data plan, they can be converted to an unlimited LTE data plan. AFAIK, a new SIM card is required for LTE data.
I think that AT&T still offers unlimited data for feature phones, but there is no unlimited data for smartphones still offered. Even when AT&T did offer unlimited data for smartphones, it was $30/month.
I don't know of any feature phones that use LTE data, so the answer to your original question would be no.