systemd-stable/extras/run_directory/README

Use these binaries only if you need backward compatibility with
older udev versions. The use of /etc/dev.d/ is no longer recommended.
Use explicit udev rules with RUN keys to hook into the processing.

  /etc/dev.d/ + /etc/hotplug.d/ directory multiplexing is completely
  removed from udev itself and must be emulated by calling small
  helper binaries provided by these helpers:
      make EXTRAS=extras/run_directory/
    will build udev_run_devd and udev_run_hotplugd, which can be called
    from a rule if needed:
      RUN+="/sbin/udev_run_hotplugd"
    The recommended way to handle this is to convert all the calls from
    the directories to explicit udev rules and get completely rid of the
    multiplexing. (To catch a ttyUSB event, you now no longer need to
    fork and exit 300 tty script instances on bootup you are not interested
    in, it is just one rule that matches exactly that single device.)