To get titles of applied rules

add_action( 'woocommerce_checkout_order_processed', 'addDiscountTitleToOrderNotes');
function addDiscountTitleToOrderNotes($order_id) {
    $applied_rules_title = array();
    if(class_exists('\Wdr\App\Controllers\ManageDiscount') && class_exists('\Wdr\App\Controllers\DiscountCalculator')){
        if(!empty(\Wdr\App\Controllers\ManageDiscount::$calculated_cart_item_discount) && !empty(\Wdr\App\Controllers\DiscountCalculator::$rules)){
            $applied_discounts = \Wdr\App\Controllers\ManageDiscount::$calculated_cart_item_discount;
            $rules = \Wdr\App\Controllers\DiscountCalculator::$rules;
            foreach ($applied_discounts as $applied_discount){
                if(isset($applied_discount['total_discount_details']) && !empty($applied_discount['total_discount_details'])){
                    foreach ($applied_discount['total_discount_details'] as $rule_id => $total_discount_details){
                        if(is_string($rule_id)){
                            foreach ($total_discount_details as $price_rule_id => $total_discount_detail){
                                if(isset($rules[$price_rule_id])){
                                    if(!isset($applied_rules_title[$price_rule_id])){
                                        $applied_rules_title[$price_rule_id] = $rules[$price_rule_id]->rule->title;
                                    }
                                }
                            }
                        } else {
                            if(isset($rules[$rule_id])){
                                if(!isset($applied_rules_title[$rule_id])){
                                    $applied_rules_title[$rule_id] = $rules[$rule_id]->rule->title;
                                }
                            }
                        }

                    }
                }
            }
        }
    }

    //$applied_rules_title - Here you get titles of applied rules
}