[jasomill]

As far as I know, all z/OS ancestors were capable of running on non-IBM systems, and so was z/OS itself until it stopped supporting the 31-bit ESA/390 architecture which was the last IBM mainframe architecture successfully cloned by any of the plug-compatible manufacturers.

MVS 3.8j was, however, the last version of MVS that was both available from IBM without a license and which lacked copyright protection under which IBM could prohibit unrestricted use.

And, to be perfectly pedantic, a number of z/Architecture "clones" now exist in the form of software emulators — including at least one[1] developed and sold by IBM itself — and these are perfectly capable of running more recent z/OS releases, possible license complications notwithstanding.

[1] https://www.ibm.com/products/z-development-test-environment

[rbanffy]

True. If you start today, you are limited to 3.8j, because IBM won't license z/OS to you. If you already have a license, you can continue using your software on non-IBM hardware, but I think IBM explicitly limits your ability to run z/OS the same way Apple does macOS.

As for the ZDTE, we can call it "virtual IBM hardware" ;-)