Doesn't this apply to all the legacy carriers (DL, UA, and AA)? My understanding is that this is a legacy of the old computer reservation systems that allowed travel agents to cancel a ticket up until midnight the next day even before the DoT rule that required the airlines to offer this to all passengers.

As mentioned, just be careful on what midnight is ... I know with my corporate travel system if I book a ticket this morning for a trip, I can cancel it any time before tomorrow night, e.g., I'm waiting for a client to confirm everything is set but want to get my ticket booked.