Official comment
The DDJ-800 was originally launched when rekordbox 5 was out, and it required a license key. Since then, rekordbox 6 was launched, which uses a hardware-unlock method instead. You don't need to purchase anything extra unless you want access to the additional features in the Creative plan (which you could always test on a 30 day free trial after you've got a feel for how the base features work for you).