I received the email saying my self-hosted Ubuntu server used TLS-SNI-01 on 2018-11-28. I followed the link to the instructions on this site, but could not find anything wrong with my server. Certbot version is 0.28.0. There is no mention of tls-sni-01 in my config files. The results of certbot renew --dry-run indicate all renewals succeeded using http-01 challenge.
Neither the email nor the pinned topic here reflect this situation. It appears the email was generated in error, or the Ubuntu PPA (updated 2018-12-14) has already solved this problem.