As far as I know, and I'm a pretty recent NHL fan, way back it was a 10 minute sudden death OT. In mid 80s it was changed to 5 minute sudden death OT. In 2000, they went to the current 4 on 4 OT for regular season. Playoff OTs are still played with each team at full strength.
In mid-2000s, they added the shootout at the end of OT, and no longer allowed regular season ties.