I suggest we keep this thread to v7, which has pretty straightforward semantics for PostgreSQL. v8 by definition has many possible implementations, so you're going to have to make pretty strong arguments that yours is the best and only one, if you are going to claim the gen_uuid_v8 function name.
Thanks Peter, I'll follow this course of action.
After discussion on GitHub with Sergey Prokhorenko [0] I understood that counter is optional, but useful part of UUID v7. It actually promotes sortability of data generated at high speed.
The standard does not specify how big counter should be. PFA patch with 16 bit counter. Maybe it worth doing 18bit counter - it will save us one byte of PRNG data. Currently we only take 2 bits out of the whole random byte.
Best regards, Andrey Borodin.