The Norwegian page is slightly more helpful. It says that onboard announcements will be given on flights where passengers with checked luggage should use Connecting Norway. It certainly was announced on my ARN-OSL flight on Friday.
On WX HEL-OSL, it has never been announced, but you are right - yesterday on ARN-OSL with SK mainline it was announced. Consistent inconsistency again.
Incidentally, while I've gone INT-DOM at OSL maybe 20 times since the start of the year HBO, I'd like to add my first ever experience of Connecting Norway with a checked bag - it was terrible. So perhaps a longer trial period is necessary
On a quiet holiday Monday evening, there were ~30 people waiting over half an hour for their bags to be 'ready' (not the "20-25 minutes" waiting time suggested). Then it took a good 5 minutes for me to reach the head of the queue (people just didn't seem to understand the user interface at all and didn't stand aside, just blocked everyone else), and there was no indication that "the system" knew that I had checked baggage - other people seemed to have to select their bags from a few pictures, but I just let straight through, as was my travelling companion. Then it was a run to the gate at the far end of the domestic side.
If they can't make Connecting Norway work consistently, and can't get it to work efficiently for checked baggage, they shouldn't do it at all.