The Xfce GitHub organization is a read-only mirror of most of the repositories of the Xfce Desktop Environment. You can use it to view the source code of the project. It is not used for contributing to the project.
Xfce has a separate tool for submitting bugs and patches. You can read all about this in the bug reporting section.
Modules are created and updated on-the-go when a change gets pushed to a repository. Some repositories are also excluded from the mirror (e.g. user repositories from Xfce developers and the archives).
For all of the active, and up-to-date Xfce repositories visit: gitlab.xfce.org.