Ubuntu 16.04 LTS is supported until 2024 through Canonical’s Extended Security Maintenance (ESM) product. Xenial enters the ESM period in April 2021, with security patches provided for an additional three years beyond the traditional five-year standard support.
(emphasis mine; taken from here)
Related BBB issue: https://github.com/bigbluebutton/bigbluebutton/issues/8474#issuecomment-802798671