We may need further registries, for example for standardized feature
identifiers that extend the small list in R7RS-small.
I would have proposed to use the SRFI process for it. Whenever there
is a major change in the registry, an updated SRFI (only containing
that registry) is issued while the previous version will be withdrawn.
These really don't qualify as extensions to Scheme, do they?
I think it is simpler or safer than inventing a new process, whose
life may depend on the lifetime of the git repo chosen.
Git repos are easily moved from place to place, preserving their history, and links to them are cheap.