Fsfc-private I agree with you that list has nothing but old spam in the archive. Therefore I have just disabled it.
It seems like it would be a good idea for the savannah interface to automatically make archives private when it creates a "non-public" list, if it doesn't already. (I know it didn't at one point, but I can't remember if that got fixed.) (assuming that these mailing list must always be associated with Savannah projects) It is possible for admins to create mailing lists directly on lists.gnu.org without going through Savannah. FSF sysadmins do this not infrequently for FSF-specific lists. Best, Karl