back then in 2010, most email clients didn't support the URL option and they only supported mailto: addresses in List-Unsubscribe header to which they would send an email when a user clicks the unsubscribe button and you'd need to handle it at the mail server level. It's only recently that the popular email clients started supporting the URL and Google and Yahoo said that they are going to make it a necessary requirement in 2025 for senders with large volumes. You can optionally add a List-Unsubscribe-Post header along with List-Unsubscribe in which case, the client won't navigate the user to the specified URL and only sends a post request to it so that anti-spam bots does not accidentally unsubscribe a user by GETting the url for inspection. Ideally, you may put both mailto: and http links in that header and process both.