Day Of Departure upgrades have indeed been there for a very long time. Even before the OLCI gave special offers to people. DOD's were interesting since you could do cash+miles upgrades. I booked Y tickets in Q and then paid 32.500 miles + €250 for the upgrade to CUR.

You could ask for it during check-in at the airport or the lounge, now it's also possible to ask for it onboard. During OLCI you only get cash upgrades offered I believe, so it's still interesting to ask for a dod yourself if you have miles to use.