Ubuntu 16.04 is going to be EOL by April 2021 and these images would no longer be supported on our infrastructure to receive updates/new builds. The version of python 2.7 that is available on Ubuntu 16.04 is already EOL and does not works with some of the tox/verify jobs. Therefore, it's required to update existing jobs to use the Ubuntu 18.04 image and decommission Ubuntu 16.04 images.
The below Ubuntu 16.04 images are in use in the releng/builder repo CSIT jobs: