Well, the version that Digital Ocean installs has ifconfig. hostname -i on the Digital Ocean droplet I just checked returns 127.0.1.1, so that doesn’t seem helpful.
I don’t remember any in the past 20 years. Apparently, it’s been deprecated. It seems that ip -a is the New Way.
It worked on Centos, which now appears to work with the Docker that ./discourse-setup will install (alas, that script doesn’t actually start Docker, but I can probably fix it so that it will).
It could be that Digital Ocean adds ifconfig.
One or both of Debian and Centos are missing git.
I think I can do a bit of work and make ./discourse-setup work for Ubuntu, Debian, and Centos, including installing Docker and git.