OLCI is not mandatory.
I did not want to OLCI, given that I don't have a printer at home and it would have been inconvenient to go to the office on December 23rd, just to use one. Besides, I was checking bags.
If I had checked-in online after 7:05pm on the 23rd, then what ? I would have found that VS had moved my flight to the 25th. Delta could have offered to take me to London connecting somewhere in the US, that is leaving SFO in the early morning, but guess what, I purchased a flight on the 24th at 7:05pm precisely because I could not leave in the morning.

I don't think this information about how to OLCI is worthy of a FAQ, given that it is of general character applying to any delta flight and the fact that the "flying to London" FAQ has already reached biblical lengths, diluting its usefulness.