You'd be amazed at how little the underlying ecosystem has changed over the decades. Because it basically keeps compatibility forever, software that was written in the 60's can still run on modern machines. Because of this, learning using 80's software in the mainframe world is not too bad. For example, I keep a book on MVS on my desk from the 80's for reference, and most of the time it suffices for what I need to consult.

At a high level, I suppose, but for example...3.8j is 24 bit versus the current z/os 64 bit.

Indeed the last official MVS freely available is 24 bit, but then you have stuff like MVS380, which supports 31 bit and has 64 bit support in the works.

http://mvs380.sourceforge.net/

Indeed, the 16 MiB virtual memory space is kind of limiting for anything remotely modern :-).

It teaches efficiency to learn the basics on such a system, something modern programmers are often lacking

80% of what a beginner to IBM big iron would learn at first can be done on MVS 3.8.

It's more like the difference between SysV and Linux, or Windows NT and modern Windows. Major differences and lots of features in the newer stuff, but for first principles it'll do.

Like I said, if you want modern(ish) Z/OS, ADCD is your best bet.