I have made an initial Flatpak available at: https://github.com/Siosm/org.freeorion.FreeOrion
This is thus not really a packaging request and more of a call for any interested maintainer to take over maintenance from there.
Project information: FreeOrion is a free, open source, turn-based space empire and galactic conquest (4X) computer game being designed and built by the FreeOrion project. FreeOrion is inspired by the tradition of the Master of Orion games, but is not a clone or remake of that series or any other game.
License: The FreeOrion source code is licensed under the terms of GPL v2, game assets are licensed under the terms of CC-BY-SA-3.0 and game content scripts are licensed under the terms of both GPL v2 and CC-BY-SA-3.0.
Upstream has been contacted: https://github.com/freeorion/freeorion/issues/1914