Forbid the use of dependencies that use unknown or forbidden licenses.

config =
    [ NoUnapprovedLicense.rule
        { allowed = [ "BSD-3-Clause", "MIT" ]
        , forbidden = [ "GPL-3.0-only", "GPL-3.0-or-later" ]
        }
    ]

If the license of a dependency is in the allowed list, the dependency will not be reported. If it's in the forbidden, the dependency will be reported as an error. If it's in neither, the dependency will be reported but with a different message asking you to add the license to either list.