This is true, and I regret on my immatureness in software development two years ago when I had the project handed over. Also, due to the fact that the git history is empty when the project was handed to me, keeping the two branches (official and unofficial) with no common git history is a pain, especially when the two branches started to occupy same Meta-Item IDs. This is the same reason why a PR is hard, which basically require adding the stuff into the official branch manually again. Though I would say if someone has the time and effort (which I don’t have currently), it is possible to PR the Machines (MTEs in loader b), Materials (MT.java), and Recipes (RM.java and loader c) relatively easily. Considering GT6U has the same license with GT6, there should be no license concerns.
Personally I and my team are working on a new project on MC 1.16: The Winter Rescue Modpack This modpack encompasses what I considers the core innovation and playfulness in GT6: modularity, material-based technology, and energy diversity. We are working on a realistic dynamic temperature system, research tree, rotational-power based steam technology, Immersive Engineering-style industrial mulitblocks, and a custom food cooking mod centered in Stews and Soups. If you are interested, we are always at our discord: TeamMoeg Community