Hey!
Since some time reservations on the daytime trains seem to be also mandatory. My brother was using a Vy train from Bergen to Myrdal this summer and was just lucky to get away without a fine as he also thought there is no reservation mandatory… conductor said if he’d travel further he’d have to pay.
Also I saw in a German railway forum some weeks ago timetable documents from Norway where these trains were marked as reservation mandatory.
On nighttrains seat reservations were already before mandatory, although not every conductor was very serious about it.
In general trains in Norway were almost always pretty full when I used them, so a reservation would be so or so no bad investment (even if reservations would not be compulsory).