Let’s Encrypt is ending support for v1 of their ACME protocol (the thing that actually issues/renews certificates.
Full details: https://community.letsencrypt.org/t/end-of-life-plan-for-acmev1/88430
Key dates:
November of 2019: LE will stop allowing new account registrations through our ACMEv1 API endpoint. Existing accounts will continue to function normally.
June of 2020: LE will stop allowing new domains to validate via ACMEv1.
June of 2021: LE will entirely disable ACMEv1 as a viable way to get a Let’s Encrypt certificate.
First, the good news. Trellis v1.2.0 was just released and it includes ACMEv2 support which was added in this PR: https://github.com/roots/trellis/pull/1114
Existing Trellis sites should continue work for renewing certificates until June 2021. This is because if you’ve provisioned prior to Nov 2019, your account will already exist.
However, you should try and upgrade to Trellis v1.2.0 as soon as possible to avoid any potential issues renewing your certificates. If you don’t want to upgrade and just want the ACMEv2 support changes, you can apply the patch only.
Hopefully this transition is smooth 
Please let us know if there’s any issues with ACMEv2 in Trellis.