nit

The sample implementation in the manpage itself — yes, was added very recently.

But even the very first version of the sd_notify(3) manpage was clearly saying that a reference implementation, designed for vendoring into other projects (and thus written to C89 standards without any extensions), was maintained in src/sd-daemon.[ch] within systemd sources (300 SLOC, of which the majority were attributable to other functions that had no bearing on sd_notify() itself and could be left out if not needed).