Gist for adding a basic Publishing Checklists

class-publishing-checklist.php

<?php
class Publishing_Checklist {

    private static $instance;

    public static function get_instance() {
        if ( ! isset( self::$instance ) ) {
            self::$instance = new Publishing_Checklist;
            self::$instance->setup_actions();
        }
        return self::$instance;
    }

    private function setup_actions() {

        add_action( 'publishing_checklist_init', array( $this, 'action_publishing_checklist_init' ) );

    }

    /**
     * Initialize our Publishing Checklist rules
     */
    public function action_publishing_checklist_init() {
        $post_types = array( 'post' );
        $args = array(
            'label'           => esc_html__( 'Featured Image', 'demo_publishing_checklist' ),
            'callback'        => array( $this, 'validate_demo_checklist_task' ),
            'explanation'     => esc_html__( 'A featured image is required.', 'demo_publishing_checklist' ),
            'post_type'       => $post_types,
            );
        Publishing_Checklist()->register_task( 'demo-featured-image', $args );
    }

    /**
     * Validate a given checklist task
     */
    public function validate_demo_checklist_task( $post_id, $id ) {
        switch ( $id ) {
            case 'demo-featured-image':
                return has_post_thumbnail( $post_id );
        }
    }

}

functions.php

<?php
require_once dirname( __FILE__ ) . '/class-publishing-checklist.php';
Publishing_Checklist::get_instance();