Require featured Images

featured-image.php

<?php

/**
 * Require Featured Images
 *
 * @param $post_id
 */
function check_thumbnail($post_id)
{
    $post_status = get_post_status($post_id);
    if ('trash' !== $post_status && 'auto-draft' !== $post_status) {
        
        // change to any custom post type
        $post_type = get_post_type($post_id);
        if (!in_array($post_type, ['post'])) {
            return;
        }

        if (!has_post_thumbnail($post_id)) {
            // set a transient to show the users an admin message
            set_transient('has_post_thumbnail', 'no');

            // unhook this function so it doesn't loop infinitely
            remove_action('save_post',  __NAMESPACE__ . '\\check_thumbnail');

            // update the post set it to draft
            wp_update_post(['ID' => $post_id, 'post_status' => 'draft']);

            add_action('save_post', __NAMESPACE__ . '\\check_thumbnail');
        } else {
            delete_transient('has_post_thumbnail');
        }
    }
}
add_action('save_post', __NAMESPACE__ . '\\check_thumbnail');


/**
 * Check if the transient is set, and display the error message
 */
function check_thumbnail_error()
{
    if (get_transient('has_post_thumbnail') == 'no') {
        echo '<div class="error notice notice-error is-dismissible"><p>'. __('You must select Featured Image. Your Post is saved but it can not be published.') .'</p></div>';
        delete_transient('has_post_thumbnail');
    }
}
add_action('admin_notices', __NAMESPACE__ . '\\check_thumbnail_error');