This is a recent notification from letsencrypt.com service:
We’re writing to inform you that we intend to discontinue sending expiration notification emails. You can learn more in this blog post. You will receive this reminder email again in the coming months:
Ending Support for Expiration Notification Emails - Let's Encrypt
It would be helpful if Let’s Encrypt HA add-on would implement a feature to send a (preferably configurable) HA notification a number of days before the certificate expires. This would enable users that are not using automated renewals (e.g. user not wanting to have a router port constantly open) to get notified in due time without risking the loss of access.