"...this is interesting is because POWER9 is basically the first time the public got a real view of how sophisticated the backstage cast actually is of a modern server CPU."
Not quite correct; the OpenSPARC T1 and T2 were publicly released and available by 2008.
https://www.oracle.com/servers/technologies/opensparc.html
"Large parts of this process are handled by vendor-supplied mystery firmware blobs, which may as well be boxes with “???” written in them.
The maintainers of the me_cleaner script likely have the clearest view of what is known.