I followed RISC-V relatively closely when it was more or less getting started still (ie it was still just a spec, there was no hardware yet).
If I remember correctly, (at least in the early stages) there were some CPU designs being made Open-Source / Libre as well, but they might well be niche / poor performance.
I recently thought about open-hardware again, and it came to me that of course AMD_64 is an open specification as well, I guess ? (But of course, there are no CPU's with Open designs for that either...at least not in the "performative and available" segment :P ).