It looks like there are some AGPL copies on Software Heritage, such as https://archive.softwareheritage.org/browse/origin/directory..., which mysteriously has gone missing from GitHub itself.

There are a few different open-source finite element solvers available, but I don't know enough about them to know what their strengths and weaknesses are. Coincidentally, over the last day, I've been vibecoding a multiphysics engine for sand battery design... on a grid rather than using FEM, although I'm starting to wonder if FEM would start to pay off at this point, because a thin fiberglass blanket around the outside of the sand battery seems to make a big difference, but reducing the grid grain to the thickness of the blanket would suck pretty bad.