Yes. It's best to contrast this versus say ARM where everyone has to pay for the ISA and potentially they also license an implementation or partial reusable designs. Sparc, Risc-V, MIPS and PowerISA are royalty free ISAs with open source implementations.
The ISA and implementation of POWER has been license-able from the beginning. You can still license implementations, depending on what you are doing that may be pretty cost effective.
Hiding in the details of your comment and partially in reply to the parent...
You can't fab the same POWER9 that ships from IBM, but like you implied, someone could (eventually) fab a POWER9 CPU built on an open-source core.
The microwatt project [0] was demoed at the OpenPOWER Summit and is an open-source softcore. I'm very interested in watching to see what happens with this project and how much of the ISA they end up implementing. They've run a subset of micropython [1] on it and apparently gotten a few FPGAs working. The Summit talk [2] mentioned DRAM support and Linux support potentially being on the roadmap.
As far as I'm aware, the entire POWER9 design is available for licencing for $$$.
The only obstacle to fabbing it yourself would be that it's designed to be fabbed on GF's 14HP process, which incorporates some IBM-proprietary fab technology which was transferred from IBM when they spun off their own fab to GF; thus AFAIK the GF 14HP process is only licenced for IBM's use. Since IBM is willing to licence POWER9, perhaps they'd also be willing to negotiate access to the process.
(GF 14HP is different to GF 14LPP, which is what other GF 14nm customers (e.g. AMD Zen) use.)
Right?