Defacto2 has a downloadable daily dump of the MySQL 5.7 database.
Defacto2 has a GitHub organisation where you can find notes and wikis for Defacto2 and repositories for historical scene websites. As well the defacto2.net repository where you can submit issues and feature requests for this website.
Is closed and unavailable
Currently, the source code to Defacto2 is not open, but a longer term goal is to have it publically accessible on GitHub.
There is no API available