Where is twmrc.system?

According to the twm() it is in /usr/local/share/X11/twm/system.twmrc, but it is not there.

As far as I remember, this file existed, because I edited one to personalize it.

BTW, there is an inflation of bloated or broken WM, and few meager alternatives. fvwm is not anymore usable. It remains twm and cwm.
The file is indeed there. But the man page mentions at the end another file that do not exist: /usr/local/lib/X11/twm/system.twmrc.

Perhaps is a correction of the man page necessary.

In the source are some sample configurations, I think they should be installed somewhere.
Useful tip, use pkg-info(8), for example: pkg info -l <packagename>. This will show you a list of files contained in the package.

     -l, --list-files
             Display all files installed by pkg-name.

Another useful one is pkg-which(8):
# pkg which /usr/local/bin/tmux
/usr/local/bin/tmux was installed by package tmux-2.5_1