As a hobbyist without a lot of time for sysadmin, it would be nice if basic email monitoring was a standard package (apt install letsencrypt-monitors or something).
Just use certbot. It automatically sets up a scheduled task to renew your SSL/TLS certificates in the background, typically using a systemd timer that runs twice a day. I do not know why people using LetsEncrypt would not set up certbot along with it, that is how I do it. Some nginx config + certbot.
Maybe the situation's improved, but I found certbot from system package managers would diverge from latest version, sometimes significantly, like support for DNS challenge APIs breaking. I switched to ‘acme.sh’ for most machines and haven't looked back. It no longer has Let's Encrypt as its default issuer, but you can set it back to LE with one config command.
You can get pretty close to this by (1) setting up certbot and (2) configuring your system to actually send emails if cron jobs fail.
I can see the use in a tool that will scan all certificates configured in local web servers and monitors for close expiration dates, though. Not just Let's Encrypt, but also any other ACME accounts and certificate directories you may need. The biggest challenge would probably be dealing with encrypted certificate files, and after that getting email set up correctly. Nobody seems to have made it because it's so easy to script or add to a pre-existing monitoring system, so this could be a fun open source project. You probably can't use the letsencrypt brand name, though.
