I noticed this on a recent flight, MSP-SFO.
Choices were 60-minute pass ($10), monthly pass and annual pass. Maybe something else to. If you clicked through, you could get a "prepaid 24-hour pass" for $18.
On my first flight, I thought prepaid meant, have you paid us money already. So I bought two 60-minute passes for a total of $20. On the way home, I pre-bought through delta.com a 24-hour pass for $12. I also learned from my boss (on the same route, but an earlier flight) that you can get the prepaid 24-hour pass" and use it immediately.
So, to summarize, it appears the flight passes are going away, to be replaced by 24-hour passes. What will be really interesting is if Delta sells the 24-hour passes in advance at a dynamic price based on your segments or total flight time. And, if you think you're going to use the WiFi, it's generally best to buy in advance. As far as I can tell, the 24-hour pass isn't tied to the flight you buy it for.