I'm not sure which udev update this "broke" but now my /dev/input/by-path or /dev/input/by-id is not being populated for a PS/2 keyboard. I looked at the udev rules file at /lib/udev/rules.d/60-persistent-input.rules and found that udev will populate the /dev/input link if I comment out the line regarding: SUBSYSTEMS=="serio", ENV{ID_BUS}="i8042", GOTO="persistent_input_end" however this does not feel like the proper fix or correction.
Please tell us which version of sys-fs/udev you happen to use.
The relevant change seems to be this: https://github.com/systemd/systemd/commit/f013e99e160f385a0c02793c612ef4c8a8ffc4d7 I suggest you open an issue upstream: https://github.com/systemd/systemd/issues/new
The udev version is 217. I will try to push upstream, thanks.