Seriously, what's the point?
Having this on a theories page is practically pointless. Just because the show's over doesn't mean that most of the theories are invalid. Some of the archived theory pages do have interesting gems and ideas in them, and the OBE policy seems entirely counter-intuitive.
Leaving the theories pages visible, without forcing users to go through the history, is a much better option than just hiding it all with a green box.
I don't see anything wrong with leaving them open for continued discussion and speculation, since it's one of the few things left that would logically continue to generate interest. If the plan is to hide theories behind an archive template, then why even leave them up in the first place?
Overall, having the pages "kept for historical reference purposes" serves only to eliminate whatever ideas people have come up with, and belongs to a "it's over, so end it" mentality.