The current ebuild forcibly removes the default initscript `/etc/init.d/ifplugd` in favor of default handling by `net-misc/netifrc`. I find that odd, since people _may_ want to opt for a different behavior, closer to what was originally intended. In that case, having access to the original initscript could prove convenient.
Unfortunately, that default behavior renders `ifplugd` completely useless for my target scenario, since bringing down the wired interface also stops `ifplugd` on said inteface; but the whole point of using `ifplugd` would be to bring that interface back up, once the cable is [re]connected.
I tried disabling the relevant `netifrc` module by setting `modules='!plug'` in `/etc/conf.d/net` to no avail. (See separate bug report: https://bugs.gentoo.org/761987.) As long as the PID file `/var/run/ifplugd.eth0.pid` exists, `netifrc` will stop `ifplugd`.
The only workaround I found was to manually delete the PID file. That stops `netifrc` from interfering, and let's `ifplugd` keep running.