I've recently looked again into the strange world of IBM operating systems (z/OS, IBM i, AIX), primarily because they all run Java, and my open source library junixsocket (
https://github.com/kohlschutter/junixsocket) needed a corresponding JNI (C code) implementation on these platforms for the native part of supporting UNIX domain sockets in Java. Of these IBM OSes, I found that z/OS was the weirdest one.
Since you're an enterprise consultant already, you probably already know that there's always an opportunity to cut down expenses/technical debt, while providing justification for past decisions made by the company (after all, they've probably invested quite a lot in that infrastructure, so these decisions couldn't have been wrong...)
Depending on how deep down the rabbit hole you dare to go, there may be a pot of gold waiting for you or a black hole that sucks the livelihood out of your body. After all, it's mostly a legacy system. Don't expect entirely new systems being written for those niche operating systems. But maybe you find your very own niche in there.
I know a company that I worked with in the past has their main business powered by IBM mainframes, and they may well keep that system going for the foreseeable future. A lot of code may already be running on zSystems Linux or be converted to run on it. At that point, you largely only have a different processor architecture to deal with, and some minor, yet annoying incompatibilities and restrictions that you will encounter soon enough.
A good starting point to learn about IBM z may be "IBM Z Xplore" (https://ibmzxplore.influitive.com/), which gives, apart from an online tutorial with challenges, some free access to IBM z systems. Also check out IBM ZD&T for Learners Edition (https://ibm.github.io/zdt-learners-edition-about/).
Don't expect that IBM responds to any of your inquiries unless you already are a paying customer.