Earlier quoted context omitted.
They've restricted it slightly: now you have to notify them at least 10 minutes before the flight. But still no penalties - I really hope they don't have to do away with that policy since that's the main thing I like about Southwest.
The 10 minutes prior departure cancellation rule is for full refund to the original payment method, i.e. refund to credit card or cash. If you missed the flight without cancellation, you got back SW credit as refund, which has to be used within a year.
After that, you get nothing. It's in the contract. I think it's only been that way for 2 years or so.