I don't think I recall an entire cruise completely selling out on opening day, but there may have been a transatlantic a few years ago that came close (maybe it did sell out, I wasn't really paying close attention that year). There have been times that some categories sold out, and times that several reach GTY on opening day. "Unique" itineraries do sell quickly among CC members.
I read someone mention they received a GTY room assignment about a week after opening day; if
DCL goes through and assigns all GTY and still has considerable inventory, they may put it back out to select a specific room. I suppose it may depend how many holds drop right away. I'm just not thinking huge amounts of rooms will drop back into inventory tomorrow or early next week - I wouldn't be surprised if many did actually put down a deposit to buy themselves time to decide. Whether the party will ultimately cancel before PIF is another question.