Comment by Joker_vD
How would knowing the full path help you anyway? It's either in "/usr/bin/sl", or "/usr/local/bin", or "~/.local/bin", now what?
By the way, believe it or not, POSIX compliance requires existence of only two directories (/dev and /tmp) and three files (/dev/console, /dev/null, and /dev/tty) on the system; everything else is completely optional, including existence of /bin, /etc, and /usr.
Because you know what you installed and so which sl to use.