I am trying to install PostgreSQL 16 on Ubuntu 18 and added your repository deb https://apt.postgresql.org/pub/repos/apt bionic-pgdg main but it seems that version 16 is not available. Tle latest available version is 15. Is there some problem with the new version and why is it marked as beta in your wiki https://wiki.postgresql.org/wiki/Apt since it is already released?
18.04 is EOL so my guess would be that they are no longer creating packages for that release.
As to the wiki (and/or any other references), small number of people, doing way too many things - some stuff doesn't get updated as quickly as one would like.