The way we got this is interestingly twisted.
French citizens requested the software used by the administration, and they managed to get it!
But, it was written in Mlang, a proprietary language created by the french administration in the 90: https://github.com/MLanguage/mlang.
Someone then decided to create an OCaml compiler that takes mlang and emits python: https://arxiv.org/abs/2011.07966
As a result, we got Open Fisca. I now think there are other techs in the mix...
A slide of the story: https://www.slideshare.net/Etalab/opening-up-the-french-tax-...
But I believe it should be mandatory for the state to provide the source of every softwares they use internally to manage our lives. Also a test suite to allow any citizen to validate their own effort to comply with the law.
Just like laws should be published as diff of the previous laws, in a VCS repo.