Removing all Category pages with one exception

You can just change the if (is_category()){ line:

// Remove category archives
add_action('template_redirect', 'jltwp_adminify_remove_archives_category');
function jltwp_adminify_remove_archives_category()
{
    // Ignore category 20.
    if (is_category() && !is_category(20)){
        $target = get_option('siteurl');
        $status="301";
        wp_redirect($target, 301);
        die();
    }
}