The Ethereum Foundation (an open-source project) will build an open-source ASIC to support its decentralised randomness beacon.
To date, has the RTL of any ASIC been open-sourced or will the Ethereum Foundation set a precedent?
Short answer: No. It is not a precedent.
So in short, OpenAnything is not new. As many other ideas, like the Internet or the web, it actually starts from academic collaborations that later became inclusive of the whole public and took a life of their own.
So no. They will not be the first. And no. You cannot have "OpenSource ASICs." That simply does not make sense.
The Ethereum Foundation can simply do what all of us mere mortals do and grab and OpenSource RTL and load it into an FPGA. But that will not be an "ASIC."
The Ethereum Foundation can contact one of the many fabless houses or fabs that offers the full service of RTL-to-ASIC to do that part of the work for them, but that would not be "OpenSource."
But if that is not enough, here is some people that went all the way to ASIC, released what was basically a single hobbyist labor of love as a product, and then went back to FPGA so that it could actually be released as OpenSource. So no, they will not be the first.
"manufactured ASIC" is a compilation of RTL (which can be open) and particular FAB-supplied libraries of basic elements, and usually third-party I/O pad libraries, for which you need to pay big dollars. These are properties of FAB houses, and they differ in time and evolve with process node. The RTL can be "open sourced", but you need to work hard to assemble the rest (translate, place, route, generate photomasks, etc) . And again you have to pay big dollars for software tools to do all this stuff. And pay a lot to seasoned engineers who manage the whole process. The whole premise of "open-source ASIC" doesn't make sense.
Well, there's all the stuff at OpenCores.
However, in my opinion there's no direct equivalent to "Free Software" for hardware, because the freedom to modify it yourself as an end-user doesn't really exist. You can't sensibly get them manufactured in single quantities. Much of the tooling is closed-source, and the technology libraries are usually proprietary to the foundry.
Sun open-sourced the code for the UltraSPARC T1 in 2006 and the code for the UltraSPARC T2 in 2007. Both were released under the GNU General public license v2.
The OpenCelerity project manufactured a TSMC 16nm "Open-Source RISC-V Tiered Accelerator Fabric SoC".