Interestingly, I was on a flight from NRT to HKG in Y, which was nowhere near full (Expertflyer was showing Y9 all the way to Q9).
Someone managed to select the seat next to mine pre-check in (not sure if they still do soft seat block for DM). At the gate, I asked to move and checked if the seat next to me can be blocked. Was flatly rejected ("it is not possible to block seats" and "we cannot guarantee an empty seat next to you".)
In a turn of events, the FP noticed that I had moved seats last minute and offered an entire empty row.