It looks just like
FujiUI and I'm fairly certain was at least inspired by it if not directly derived from it (and possibly something from Twin2/Nin who I was friends with and probably derived one of his many excellent UIs from FujiUI as well.) Personally, I created the frames in Pitbull and used a lot of complicated and very finicky scripts to get the functionality I wanted -- most of which can easily been done now with Stuf or something similar.
I still have all my old copies of FujiUI. I've thought about recreating it over the years and even managed to get a facsimile of it going at one point with Skullflower's UI/ElvUI as the base but I've never been able to get a good class color addon going that changed everything to my satisfaction. I may work on it this expansion.
Edit: I was right, according to
this thread it was a cross of Nin's UI and mine.