systemd sysv init compatibility mode: how it works and troubleshooting when it breaks

systemd sysv init compatibility mode is magical. That is in the sense that it tries to handle compatibility with sysv init scripts while you are distracted looking somewhere else.

When it works it works well, but when things break it makes troubleshooting more difficult. Especially if you don't understand what's going on behind the curtain.

The first thing you need to understand is that this probably doesn't do what you expect: