As of about 10 years ago they still made those reference cards. And you're right about it making more sense than the 8086 if others are interested what you want to look for is the Principle of Operations document. I could only find the one for z9[1] but z13 is the latest mainframe out and there should be some somewhere.
Another interesting thing is that while you can technically run a version of Linux on these mainframe, most of them run z/OS with Unix System Services. So it acts somewhat familiar but you're sometimes forced to boot up a 3270 terminal emulator to connect and do some non-unix things.
The latest one is for z14 [1] and is a very well organized document for compiler writers. I reference it daily while working on the Eclipse OpenJ9 project. Indeed from day to day we mainly work on Linux (Ubuntu 18.04) on our z14 mainframes.
Another interesting thing is that while you can technically run a version of Linux on these mainframe, most of them run z/OS with Unix System Services. So it acts somewhat familiar but you're sometimes forced to boot up a 3270 terminal emulator to connect and do some non-unix things.
[1] https://www.ibm.com/support/libraryserver/download/dz9zr006....