The MC68000, for example, is over 40 years old and out of patent. It’s been repackaged on a smaller process as the 68SEC000, and there are Verilog implementations available too.

And that’s just the example that’s top of mind. There’s a whole line of low frequency Atmel processors too, but those are still very much in patent and so mostly tangential to this conversation.

RISC-V doesn’t really make sense for simpler implementations though; you’d still have to do a bunch of work to simplify it, AND end up with an architecture very few are currently familiar with.

Once Chinese implementations of RISC-V become endemic and there are enough people familiar with the architecture, it might make sense to start creating custom subsets on simplified processes. But we’re still years away from that.