This controller/pilot interface is called CPDLC and is used among other things by aircraft to provide position reports to controllers without communicating this by voice on HF. This drastically reduces the amount of HF communications you'll hear from aircraft equipped with CPDLC.
But even with CPDLC, pilots still need to call on HF for a radio check and a selcal check when they first enter oceanic airspace. They will aslo provide their "exit" point which basically is the next airspace they will enter. In the case of eastbound oceanic flights, they'll say something like this: "Gander, Gander, Luthansa 401, CPDLC, Shanwick is next." After that they'll be told that "no voice reports are necessary" and also "VHF frequencies will be assigned by CPDLC" if that is the case. Or they'll be told "At 30W call Shanwick on 2899".
When the flights reach the next airspace, Shanwick for example at 30W, then they'll come on HF once again for a radio and selcal check just like before.
Has the number of transmissions gone down? I'd say yes, but at the same time I believe that the number of overseas flights has gone up over the years. There's still a lot of action on HF; still worth listening to