Alright, but they're designed for you to read through config.h (note that these settings aren't inlined) and adjust them for your system before building a binary. They're very explicitly not designed for a "grab your vendor binary and you'll be fine" model. It's a different paradigm.
EDIT: The programs themselves are intended to be written in a programmer-modifiable way (they host a list of common "plugins" in patch format). I think the last thing you can say about systemd is that it encourages you to jump in and modify its behaviour to suit your needs. That's not a Supported Configuration(tm).
Depends on what you modify but it's to my knowledge supported if your patches didn't cause the bug or behaviour problem. Plus there is plenty of ways to configure systemd at compile time, you can toggle a lot of switches in systemd.
EDIT: The programs themselves are intended to be written in a programmer-modifiable way (they host a list of common "plugins" in patch format). I think the last thing you can say about systemd is that it encourages you to jump in and modify its behaviour to suit your needs. That's not a Supported Configuration(tm).