Apply a discount rule only for downlodable products. Exclude the rest

downloadable_rule.php

function woo_discount_rules_exclude_cart_item_from_discount_method($excluded_products, $rule, $variants){

	$eligible_rule_id = 9999; // enter the rule ID that applies for downloadable product only

	if(isset($rule->ID) && $rule->ID == $eligible_rule_id ) {
	
		$query = new WC_Product_Query( array(
			'downloadable' => false,
			'return' => 'ids',
		) );
		$products = $query->get_products();
		if(!empty($products) && is_array($products) && count($products)) {

			
			$all_products_to_exclude = array_merge ($excluded_products, $products);
			foreach($all_products_to_exclude as $product_id) {
				$product_ids_to_exclude[] = intval($product_id);
			}
			$excluded_products = $all_products_to_exclude;
		}
		
	}

	return $excluded_products;
}
add_filter('woo_discount_rule_products_to_exclude', 'woo_discount_rules_exclude_cart_item_from_discount_method', 10, 3);