Create a WordPress Custom Post Type

wp_custom_post_status.php

<?php
function custom_post_status()
{
    $slug = 'unread';
    $label = _x('Unread', 'post');

    // check if status already exists
    if (post_type_exists($slug)) {
        return;
    }

    register_post_status($slug, [
        'label'                     => $label,
        'public'                    => true,
        'protected'                 => false,
        'private'                   => false,
        'exclude_from_search'       => false,
        'show_in_admin_all_list'    => true,
        'show_in_admin_status_list' => true,
        'label_count'               => _n_noop(
            "{$label} <span class=\"count\">(%s)</span>",
            "{$label} <span class=\"count\">(%s)</span>",
        ),
    ]);

    add_action('post_submitbox_misc_actions', function ($post) use ($slug) {
        echo "<script>jQuery('#post_status').append('<option value=\"{$slug}\">{$label}</option>\');";
        if ($post->post_status === $slug) {
            echo "jQuery('#post-status-display').text('{$label}');";
            echo "jQuery('#post_status').val('{$slug}');";
        }
        echo '</script>';
    });
}
add_action('init', 'custom_post_status');