Kirki is licenced under the MIT Licence. It is 100% open-souce and you can use it on your projects.
Though not legally necessary, we would appreciate if you mention in your project's page that use Kirki and link to this project.
Support is provided from the github repository. Though we do our best to reply to everyone in a timely manner and address any questions or issues you might have, keep in mind that Kirki is an open-source project maintained by people donating hundreds of hours.