Why isn't there a openjdk-8-jdk package on debian anymore?
You are on Debian 10 (buster), but the link you showed is for Debian 9 (stretch).
OpenJDK is on version 11 for Buster and OpenJDK 8 is not available.
If you need this, use a docker image based on Stretch.
Apparently Debian 10 alias Buster only provides openjdk 11 packages.
In my case this meant using debian 9 alias stretch instead. I must admit it wasn't very clever using latest as a version in the first place.
Debian stretch is a LTS version supported until 06.2022