Even here on HN, one need not look far to find someone who resents everything about systemd and insists on using a non-systemd distro. Such people run systems in real life, too.

Should you run into such a system, you're still just writing code, and interacting with a daemon that takes care of the hardest parts of init scripts for you.

There are no pid files. There are no file locks. There is no "daemonization" to worry about. There is no tracking the process to ensure it's still alive.

Just think about how you would interact with the docker daemon to start, stop, restart, and probe the status of a container, and write code to do exactly that.

Frankly, Docker containers are the simplest thing you could ever have to write an init script for.

Do you have evidence that they run real systems?

Evidence I'm comfortable citing in public? No. Experience suggesting that at least one such person runs at least one real system? Yes.