I really like vintage retro (is that redundant?) computing. It’s fun, to a point, to relive past experiences. However IMHO, only MVS 3.8J on IBM S/370, on up is still actually useful today apart from nostalgia. And that would be TK4- or TK5 running on the Hercules emulator. TK5 is probably the road forward. Because of IBM’s vision of the IBM 360, way back in 1964, and new models that built upon it. Much of what the old MVS does is very similar to z/OS. Based on the same but enhanced instruction set. Many of the utilities even back to MVT are the same as MVS 3.8J and z/OS. Thanks to the work of many improving upon it.
Is running retro Apple or TRS-80 fun? Yes. Is it useful in the real world? No! Same can be said about RSTS/E, RSX-11M or RT11. But I believe you can learn a lot and improve your IBM mainframe knowledge using Hercules emulation and TK5. Because the IBM mainframe experience is very different from what most people know about computers. They don’t even speak ASCII, natively.
I loved my time with DEC PDP-11 computers. I wish DEC was still around and relevant. I never worked with VAX, but I wanted to. But unfortunately… DEC is no more!