The author has created a free service that monitors the expiration of TLS certificates via RSS feed, without requiring any sign-up. Notifications are sent out 30 days, 7 days, and 1 day in advance of the expiration date. The service only checks the “Not After” field of certificates, so even self-signed certificates will work. There are no guarantees given, but the author has plans to expand the service to monitor TLS certificates based on certificate transparency logs in the future. Readers can subscribe to the author’s blog via RSS to stay updated on these developments.
https://raphting.dev/posts/monitor-rss/