It’s not simply a matter of updating SpamSieve, since Postbox 6 has removed support for add-ons entirely. I have been talking with the Postbox folks about this, and they are advising that in the short term customers who rely on add-ons should continue using Postbox 5. Longer-term, they want to build the functionality from key add-ons directly into Postbox. Of course, we are eager to help them do this with SpamSieve, but the timetable is up to them. (You could tell them if this is important to you.) Understandably, their initial focus is on the core of Postbox, which has just undergone a massive update.
If you want to use Postbox 6 today, and still get SpamSieve filtering, one option is to install SpamSieve for Apple Mail and let Mail run hidden in the background. If you use the drone setup, you can train SpamSieve from within Postbox by moving messages into the special TrainSpam and TrainGood mailboxes, so you don’t actually have to interact with Apple Mail.