Fill in the custom_label_0 with new if the product is created within 10 day's from now.

product_old_new.php

<?php

function product_old_new( $attributes, $feed_id, $product_id ) {
	 global $product;
	
	// Get product creat
	$product = wc_get_product( $product_id );
	$product_created  = strtotime($product->get_date_created());

	// Calculate the date 10 days ago
	// Change the number in strtotime('-10 days') to your need
	$x_days_ago = strtotime('-10 days');
	
	if ($product_created >= $x_days_ago) {
		// the product is newly created and not older than 10 day's ago
		// Change custom_label_0 to the one you need or use an other attribute.
    	$attributes['custom_label_0'] = "new";
	}else{
		$attributes['custom_label_0'] = "old";
	}
	
	// IMPORTANT! Always return the $attributes
	return $attributes;
}

add_filter( 'wppfm_feed_item_value', 'product_old_new', 10, 3 );