Among other things, having a standards body
maintain an explicitly unstandardized list seems like a conflict of goals.
IETF groups often get in conflicts between documenting existing practice
and describing desirable practice. An IETF group can do either one
of these competently as long as it only tries to do one of them, and it
clearly understands which one it's doing. But trying to do both is a
recipe for disaster.
A registry introduces a third possibility - documenting practice that
isn't either existing or desirable.