;tldr If the caps lock on your old keyboard stopped working, try to install and use Karabiner-Elements. It fixed the problem for me.
The caps lock on my beloved Apple Keyboard Extended II I use at work stopped working recently. I tend not to type in ALL CAPS often, but sometimes I have to type ridiculous acronyms and caps lock is the key I need. Later I noticed it wasn’t working on the one I use at home.
There are several breakpoints, so I first chalked it up to hardware failure. These things are more than 26 years old after all.1 They also require a Griffin iMate adaptor to convert their ADB connections to USB. After I saw the problem on both keyboards, I knew something was up.
One of the updates to macOS Sierra 10.12.x broke it. This was a problem, but it was a software issue and software can be fixed. I found Karabiner-Elements fixed my problem. It enables user to remap their keyboard. You can do lots of cool stuff with this (like creating a super meta hyper key), but my needs are simple for now.
After installing the software, I opened the settings and set caps lock to caps lock. That was all it took to return functionality to my caps lock key. I added it to my Login Items (under Users & Groups in the System Preferences) and I’m ready to ROCK AND ROLL!