https://github.com/jenkinsci/workflow-cps-plugin/blob/master/src/main/resources/org/jenkinsci/plugins/workflow/cps/EnvActionImpl/Binder/help.jelly says:

SCM-specific variables such as <code>GIT_COMMIT</code> are not automatically defined as environment variables

However with today's stable Jenkins (2.289.3) and plugins, running this Jenkinsfile:

pipeline {
    agent any
    stages {
        stage('Stage') {
            steps {
                sh 'echo $GIT_COMMIT'
            }
        }
    }
}

does print out the Git commit, showing the documentation is inaccurate.