python-certbot-nginx (nor any python-letsencrypt-nginx) was never released on 16.04 so cannot regress. Michael didn't include it in his SRU work, so it didn't make this update and isn't expected to do so in the future either.
python- certbot- nginx (nor any python- letsencrypt- nginx) was never released on 16.04 so cannot regress. Michael didn't include it in his SRU work, so it didn't make this update and isn't expected to do so in the future either.