Styg has made a few DevLog posts about that. To sum up what has been posted here already: testing is going well, the testers have been helpful in identifying problems, the problems have been or are being fixed, and Styg will not guarantee a release date for the expansion.
That said, it looks like (from the outside, as a tester with no special privileges or information) it's getting fairly close. There's a page for it on Steam now; that's a good sign.