@evilwoodchuck - Serial activity is no more difficult than it ever was. It is just that usage of RS232 has diminished so much in favour of USB and ethernet that even having an RS232 port on a computer is unusual. Hence the need to make /etc/rc.d/rc.serial executable to be able to use a serial port in recent Slackware versions as well as the appearance of a minor config error in minicom as discussed in post #3.
The discussion in this thread is around establishing communication between a smart DCE device and a dumb DTE device. This means using a cable with appropriate jumpering that satisfies the needs of both devices, then having both devices agree on the baud rate, number of data bits, use of flow control and parity usage. This may seem anachronistic today, but it has always been the way with serial RS232. When set up correctly, serial RS232 is capable of sophisticated communication, just at glacial speeds compared to more modern protocols. Take a look at UUCP, which is available in standard Slackware. RS232 still finds use where a simple, robust and cheap communication protocol with only modest data transfer requirements is needed.

As to your question about the Cisco supplied RJ-45 (console) to RS-232, sorry, I cannot say for sure. I did once wire a RS232 serial cable so that I could use minicom in Slackware to reset a secondhand Cisco router nobody else wanted because it was password protected. But that was years ago and that unit went belly up earlier this year, flashing a lot of warning orange instead of a mix of steady and flickering green.