Since it is not advisable to delete any lists you wish to "archive" because you are no longer using them/mailing to them, perhaps it would be ideal to provide a setting for a list that can be turned on and it would prevent any new contacts from being able to subscribe to it. Like some kind of "subscription blocker."
As long as the list still exists, it's possible that a contact can get subscribed to it. This can cause issues when building segments and unbeknownst to you, you have contacts subscribed to an "archived" list that you are no longer mailing to directly, but those contacts can still be pulled into a segment and you may or may not want that. Plus, it's just messy to have old lists, not be able to delete them, and they are still able to be subscribed to. It's just not a good practice. The safest and most assured solution is to prevent contacts from subscribing to these lists.
