The assumption here is that you're starting with a Ubuntu 20.04 LTS Server or newer, rather than Desktop, base (Ubuntu Desktop deploys NetworkManager rather than systemd-networkd by default).
The goal is networking configured for static addressing using using ifupdown, rather than the newer systemd-networkd and netplan, or (for Ubuntu Desktop) NetworkManager. It basically returns networking on Ubuntu to its Debian roots.
The physical network device name for the test machine was "ens3", it will be something else on different hardware.
NOTE: I am currently using netplan rather than ifupdown on my own Ubuntu servers. I'll keep these notes updated because the configuration is close enough to Debian that it may come in handy when I finally switch to Debian.