I don't see systemd in there anywhere.
People who are set against systemd should probably better look elsewhere (although their options are pretty limited).
I'm sticking with Slack for the time being, but I've got my eye out for alternatives should the worst happen.
Now that could be a interesting read, if documented somewhere.
It might happen in the future, but as of now Slackware (and Gentoo) are those distros that don't have systemd. Some people might like it for that.
Apparently you can forgo systemd_pam (a poor naming choice, given the following) if you do not use logind. And logind is used for seat and session tracking, something CoreOS likely have no need for as it is meant to handle containerized servers.
Yet all this is in systemd the project, because systemd is the swiss army knife of Linux user space.
So yes, Slackware would likely need to adopt PAM if they want to adopt systemd.