Butane has been merged into the Ignition repository. All future development, issues, and pull requests should be directed there.
The Butane source code now lives under the butane/ subdirectory of the Ignition repository. Starting with Ignition 2.27.0, Ignition natively accepts Butane YAML configs at boot, and the butane CLI is built from the Ignition source tree.
For more information, see coreos/fedora-coreos-tracker#2006.
Butane (formerly the Fedora CoreOS Config Transpiler, FCCT) translates human readable Butane Configs into machine readable Ignition Configs. See the getting started guide for how to use Butane and the configuration specifications for everything Butane configs support.
For information on developing Butane, using it as a library, or understanding how the binaries released are built, see the development docs.