The links in our renewal notifications stopped working following the automatic site upgrade to Wordpress 5.5 on Aug 11th. What happened is that the links just arrived at the 'Wordpress Base Page', in our case /civicrm, with a meaningless “Do not delete this page. Page content is generated by CiviCRM.”
Spent some time investigating before finding the notification of a mandatory update to CiviCRM 5.26, and this did indeed sort the problem. Important Notice blog post
Posting here in case others also searching.