Mainframes have evolved over the years, from the S/360, S/370, S/390 and now Systemz. Part of the evolution is adapting to changing workloads and operating systems. Along with the z/OS operating system, you can now run LINUX in a mainframe partition or hundreds of virtual LINUX servers on a system.
In spite of the continued predictions of the "end of the Mainframe" it continues to be a workhorse for many companies. My opinion is that it's worth learning about.
Disclaimer: My comments on this forum are my own and do not represent the opinions or suggestions of any other person or business entity.