The forum is currently having
new software developed; upgrading SMF would be a waste of time. In addition to this, AFAIK theymos has a lot of self made security fixes added to this specific version. It would be a pain to transfer those over.
As far as I can tell that new forum is being developed since dawn of time. There is no information about ETA.
I am not sure how hard is to update SMF but if that process is not something insanely problematic maybe forum crew shout try it?