The New Horizons offering was made as a "proof of concept" that it could be done and was meant to encourage some of the New Horizons community to continue working out the remaining issues (like incompatible weather sky textures issues that remained...someone needs to convert the old sky methods with the newer, COAS way of sky texture layout) so that it could be played with a newer engine, just like the community supposedly wanted for the last several years. Unfortunately, only one person devoted some effort to test and help figure out those details, but he has long since abandoned the effort.
I don't intend to do that work myself, so I don't do much for New Horizons, other than to try to keep the compatibilities that I already formulated to continue working, in case a New Horizons modder wants to continue testing and helping to identify and resolve the last bits that might need work; nobody has done that. As a result, I believe the New Horizons version is still quite old, at 14 Beta 4 WIP: 31 Mar 2016.
The recent compatibility change I made for Maelstrom was to correct an internal change I had made that apparently affected only New Horizons that prevented some of the mod scripts to no longer compile. When I noticed that, I debugged and corrected that change so it still compiles the New Horizon scripts correctly, because apparently I broke that recently. But since I never play New Horizons, and don't have enough knowledge for gameplay to even test it, I did not notice right away.