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.
[0]: https://github.com/antonblanchard/microwatt