Chris, I buy your arguments that this should be an attribute. I'm not sure it should be engineered to work better as a modifier for default: or for case _:.
I think something like @unnamed might be a good choice, because the warning message could then read something like warning: default case marked @unnamed would match named enum case .newlyAddedCase; add a new switch case to handle it, or accept that the default case will handle known enum cases in the future.”
I think encouraging it to be written as @unnamed case _: might handle my earlier concern that unknown default doesn't describe an unknown default.