In wordpress there were some certain requirement if the category was deleted then we need to Trash the posts attached to theme, here is the code how to achieve this easily.

WordPress-trash-post-if-category-deleted.php

<?php
/**
 * If any category is deleted move all attached posts into trash
 */
function trash_category_posts($object_id, $tt_ids)
{
    /**
     * 1. Check the post_type
     * 2. Check the action is delete or delete-tag
     * 3. Check the taxonomy type is "category"
     */
    if(get_post_type($object_id) == "post" && ($_POST['action'] == 'delete' || $_POST['action'] == 'delete-tag') && $_POST['taxonomy'] == "category")
    {
        // Trash the post so it will be deleted after EMPTY_TRASH_DAYS
        wp_trash_post($object_id);
    }
}
add_action("delete_term_relationships", "trash_category_posts");