It was a total rewrite with new features/tools such as pming mods.
It is not perfect perhaps but I am not sure what more you would expect from a simple group system. (Besides engagement)
Anime tracking related things are the core reason why people go to Kitsu and it's competitors, so it makes sense that they would have those in high-priority.
Though they are hardly the only thing that gets worked on.
Eventually someone will remember that the forum is around and will have the time to kill it. Who knows when that will be, but it isn't really causing any issues so no real reason to bump it in priority.
PM is something that will come to Kitsu in the future though, and honestly people hardly used the forums and the forum PM so it's not something that should replace a proper PMing system.