I have yet to see one correct history in here.
You can't call, 70 minutes a "brief" period. Nor have subs ever been removed since their introduction. When they were first released the notes stated subs would be allowed in Normal Rooms, GBIs, GBIIs and Missions, they were patched incorrectly into blitz as well but where quickly removed within 70 minutes by an emergency patch.
Subs are only 2 years old, how could they have been removed for years.