A python-certbot update has been released for Debian GNU/Linux 9 LTS to replace the deprecated Let's Encrypt's ACMEv1 API support.

DLA 2484-1: python-certbot - switch to ACMEv2 API

- -----------------------------------------------------------------------
Debian LTS Advisory DLA-2484-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Utkarsh Gupta
December 07, 2020   https://wiki.debian.org/LTS
- -----------------------------------------------------------------------

Package : python-certbot
Version : 0.28.0-1~deb9u3
Debian Bug : 969126

Let's Encrypt's ACMEv1 API is deprecated and in the process of being
shut down. Beginning with brownouts in January 2021, and ending with
a total shutdown in June 2021, the Let's Encrypt APIs will become
unavailable. To prevent users having disruptions to their certificate
renewals, this update backports the switch over to the ACMEv2 API.

For Debian 9 stretch, this problem has been fixed in version
0.28.0-1~deb9u3.

We recommend that you upgrade your python-certbot packages.

For the detailed security status of python-certbot please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/python-certbot

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS