But I can direct you to Hercules which is an IBM Mainframe emulator. It is open source under an OSI approved license (not GPL but rather QPL). Their problem appears not so much that they emulate the s390 architecture (I think that the Oracle v Google Java API case set rather a conclusive precedent there, in the US at any rate, but perhaps I will be corrected), but that the license terms of IBM mostly prohibit running any of their operating systems on the emulated hardware.
Of course there are exceptions and workarounds (IBM System/360 is public domain software in the US, but MMV in other territories, and some customers have a disaster recovery contract clause that could perhaps justify their running stuff on Hercules). But my point is that this project might be a good place to start when investigating the legitimacy of commercial/proprietary emulators of others' hardware.
HTH and good luck.
IANAL, implementing the instruction sets and reverse engineering the firmware on those systems would be protected under fair use under copyright law [4]. You can be sued in civil court for copyright infringement, but you would have a valid legal defense that is supported by precedent in that case. You however could be sued if the hardware was protected by patents, but in the case of older hardware like you mentioned, patents would have been past the 20 year expiration for patent protection. You could be sued under trade secret, but the prosecution would have to show that you violated a non-disclosure agreement with them. If you never signed one, and reverse engineered the system, it would be an invalid case.
The most likely explanation for why you cannot find commercial emulators is because open source ones exist. What incentive does a software developer have to compete with a free product?
[1] https://masswerk.at/6502/6502_instruction_set.html
[2] http://z80.info/z80oplist.txt
[3] http://wpage.unina.it/rcanonic/didattica/ce1/docs/68000.pdf
[4] https://ngminusone.wordpress.com/2018/07/10/are-emulators-le...