My thought is that if we call it "Mainframes", what is currently thought of as mainframe hardware and software running on that hardware would be in scope. This includes IBM Z, the Fujitsu and Hitachi hardware platforms that have their origination from IBM System/360, Unisys, and Bull. I've written an answer over on Quora that uses this definition.
Operating systems should include the five that run on IBM Z, the Fujitsu and Hitachi ones which came from the pre-licensed IBM operating systems, BS2000/OSD (and VM2000) (Fujitsu), OS2200 and MCP (Unisys), and GCOS (Bull).
For IBM i, I think common perception is that it is still a mid-range platform. I wouldn't mind including it, but to me that is stretching the perceived definition of "mainframe".