The documentation for the check goal at https://maven.apache.org/plugins/maven-checkstyle-plugin/check-mojo.html#outputFileFormat mentions "plain" as a valid option for file output. This is not true, the mojo only supports XML.
I'd prefer added support for plain output, but the docs should be fixed if that is not possible.
Taking a look at the code, it looks like the only reason plain isn't supported is because the check mojo needs to parse the output file to figure out if there were style violations. An option would be counting errors with the AuditListener instead, but this would break support for skipExec. An alternative would be to output an extra XML result file if the output format isn't XML.
I'd be happy to implement either solution.