How to do a minimal install of 20.04?
The Netinstall/Minimal image (two names for the same thing) was a byproduct of the Debian installer server build. While popular with some users, the Ubuntu developers never supported it.
Ubuntu Server recently changed to a new installer, so the Netinstall image will longer be created. The final 20.04 Netinstall image is here.
-
For a VM, consider using a daily cloud image: https://cloud-images.ubuntu.com/focal/current/
-
For Bare Metal, consider simply using an older Netinstall image, then release-upgrading those minimal packages to 20.04, then installing the rest of your system.
-
Netbooting the server installer is still possible, see https://discourse.ubuntu.com/t/netbooting-the-live-server-installer/14510
There are many other options: Vagrant, pre-built VMs, Ubuntu Core, Ubuntu Base, etc. It really depends upon what you want your final system configuration to look like.
You still can get netboot installer (mini.iso, for now it is called Legacy Ubuntu Server Installer) from direct link below
http://archive.ubuntu.com/ubuntu/dists/focal/main/installer-amd64/current/legacy-images/netboot/mini.iso