documentation/snapshot/docs/api/custom-reporter.md
Take a look at ktlint-cli-reporter-plain.
In short, all you need to do is to implement a
ReporterV2 and make it available by registering
a custom ReporterProviderV2 using
META-INF/services/com.pinterest.ktlint.cli.reporter.core.api.ReporterProviderV2. Pack all of that into a JAR and you're done.
To load a custom (3rd party) reporter use ktlint --reporter=name,artifact=/path/to/custom-ktlint-reporter.jar
(see ktlint --help for more).
Known third-party reporters:
ktlint issues.