Addendum:

> Power ISA "3.0C"? Never heard of that. Does anybody have a link to that "3.0C" spec?

Still no link to the spec, but some mention of Power ISA 3.0C in slides from ICS in June 2020 (page 2):

"April 2020 – POWER ISA 3.0c [...]
- Same as POWER ISA 3.0b except for
• Compliancy Subsets
• Custom Extension Space (Sandbox)
• SMF Feature"

(further mentions on pages 3 and 6)

Respective presentation: https://www.youtube.com/watch?v=0zIwLCnIuqg