While trying to install PostgreSQL 10.1 from sources, I ended up looking at the /usr/lib/ directory, and there I found a systemd/ directory. I was wondering what it was useful for, since I thought Slackware-based distros actually did not use systemd. But since I don’t know much about all that, I thought I’d rather ask to understand a little bit better.
Patrick Volkerding answered a similar question not long ago, so I will just quote him:
That's it exactly. We don't gain anything (other than negligible space) by removing those files, and they would be useful to someone wanting to implement systemd on their system. If someone wants to do that, I'm not going to create obstacles for political reasons.
PS The liveslak scripts provided by Eric Hameleers can build a a Dlackware variant of Slackware which is Gnome3 + PAM + systemd on top of Slackware and stripped of KDE4.
Cf. ISO variants.