A bonus point is that Slackware does not rely on systemd. it is still a bunch of bash scripts that you can figure out what is going on.
Slackware uses a BSD-style init system, where you just have one big shell script for booting the system -- you can read through it and edit it, it's all right there, self-contained.
Additionally, all of the old Unix tomes still largely apply to Slackware. That's increasingly, and distressingly, not true of most other distros.