I definitely remember installing it off 20+ floppies onto a 386 back in the day. Most of the disks contained packages and you didn't need to install all of them though.
Early releases of Slackware were only floppy. The boot/root CD release came later. I fondly recall wasting hours in a computer lab @ university using 20 Sun Sparcstation 2 floppy drives to write the slackware floppies for me and my friend. Walking out with a brick of floppies was pure bliss (achievement unlocked).