post equivalent in scripted pipeline?

For scripted pipeline, everything must be written programmatically and most of the work is done in the finally block:

Jenkinsfile (Scripted Pipeline):

node {
    try {
        stage('Test') {
            sh 'echo "Fail!"; exit 1'
        }
        echo 'This will run only if successful'
    } catch (e) {
        echo 'This will run only if failed'

        // Since we're catching the exception in order to report on it,
        // we need to re-throw it, to ensure that the build is marked as failed
        throw e
    } finally {
        def currentResult = currentBuild.result ?: 'SUCCESS'
        if (currentResult == 'UNSTABLE') {
            echo 'This will run only if the run was marked as unstable'
        }

        def previousResult = currentBuild.getPreviousBuild()?.result
        if (previousResult != null && previousResult != currentResult) {
            echo 'This will run only if the state of the Pipeline has changed'
            echo 'For example, if the Pipeline was previously failing but is now successful'
        }

        echo 'This will always run'
    }
}

https://jenkins.io/doc/pipeline/tour/running-multiple-steps/#finishing-up

You can modify @jf2010 solution so that it looks a little neater (in my opinion)

try {
    pipeline()
} catch (e) {
    postFailure(e)
} finally {
    postAlways()
}


def pipeline(){
    stage('Test') {
        sh 'echo "Fail!"; exit 1'
    }
    println 'This will run only if successful'
}

def postFailure(e) {
    println "Failed because of $e"
    println 'This will run only if failed'

}

def postAlways() {
    println 'This will always run'
}