skalibs version 2.14.0.0 or later. It's a build-time requirement. It's also a run-time requirement if you link against the shared version of the skalibs library.
s6 version 2.12.0.0 or later. It's a run-time requirement only, to run the nsssd service (and can be done without if you have a suitable replacement for s6-ipcserver).
I love that the anti-systemd crowd is implementing their own systemd.