i'm not sure this is true. certainly "chip" IP has been a real legal quagmire since, forever.
but it was my understanding that you could neither patent nor copyright simply an "instruction set".
presumably what you get from ARM with an architecture license would be patent licenses and the trademark. if so, what patents might be relevant or would be a problem if you were to make an "ARMv8-ish compatible" ISA/Architecture with a boring name? i haven't seen much about ARM that's architecturally particularly unique or new, even if specific implementation details may be patent-able. you could always implement those differently to the same spec.
to further poke at the issue, if it's patents, then how does a RISC-V CPU or other ISA help you? simply because it's a different ISA, doesn't mean its implementation doesn't trample on some ARM patents either.
if it's something to do with the ISA itself, how does that affect emulators?
what's ARM's IP really consist of when you build your own non-ARM IP CPU from scratch? anyone have examples of show-stopper patents?