Hosting providers don't need a contract or agreement with Let's Encrypt in order to obtain and use Let's Encrypt certificates.
There is a list at
https://community.letsencrypt.org/t/web-hosting-who-support-lets-encrypt/6920/608
but this is simply informational and intended to help users choose hosting providers who are known to support Let's Encrypt. It's not a list of entities that Let's Encrypt has relationships with.
Let's Encrypt did not revoke certificates for your site. If it had, the error users saw would be totally different. Instead, your certificates expired normally on the expected schedule. Your hosting provider apparently failed to renew them. If the hosting provider obtains certificates for customers, renewing those certificates is also the hosting provider's responsibility.