Official comment
The S-series controllers are marketed and sold as Serato DJ controllers, not rekordbox. The fact that the DDJ-SZ, SX, SX2, SR, SB, SB2, DJM-S9 and DJM-900SRT were able to be added as rekordbox-compatible players is simply because rekordbox dj (performance mode) did not exist when those hardware units were created and thus we had no exclusivity contract with Serato for the software, unlike the DDJ-SZ2, SX3, SR2, SB3, and DJM-S3, which will not be available for use with rekordbox.
If you used your S-series product with rekordbox 5, you would have had to purchase a license key or subscription to use it. You are able to continue to use rekordbox 5 - we're not taking that away, but if you want to move up to rekordbox 6, you will have to register for a subscription plan, but you also still have the option to use Serato - the software for which your hardware was designed.