HaveIBeenExpired.com is a service that monitors SSL certificates and warns you when they are about to expire or have expired.
The service is currently free for unlimited domains and hosts, but the free version does throttle new host detection and is limited to 100 daily SSL checks.
The paid version is $10/month and offers unlimited new host detection and SSL checks.
The only issue I have is that my sites use Cloudflare, so the site reports back the expiration of the Cloudflare cert. The service doesn’t have visibility into the underlying Let’s Encrypt cert that the sites also use (and where I have had occasional issues with unexpected expiration).