WC Subscriptions – Create a log in WC > Status > Logs to log when a subscription has been updated.

wcs_updated_log.php

<?php

add_action( 'wcs_webhook_subscription_updated', 'wcs_updated_log', 999, 1 );

function wcs_updated_log ( $subscription ){
	
	$logger = wc_get_logger();
	$logger->info( 'Subscription ID: ' . $subscription . ' was updated! -- ', array( 'source' => 'subscription-updates-2' ) );
}