I'm not a mainframe programmer, but coming from x86 land I was very curious. I really learned a lot from the IBM Coursera "Intro to Mainframe" since none of my experience really applied it was tough. It had a real shell account to practice with though.

https://www.coursera.org/professional-certificates/ibm-z-mai...

Also the MOSHIX mainframe YouTube channel has a lot of info, and helped me setup HERCULES emulator for my own little mainframe experience.

http://www.hercules-390.org/