Goalies are so good in the NHL today that rare is the occasion when they are ever beaten cleanly on an unobstructed slap shot.
Gone, mostly, are the days when wingers would come down the wall, wind up with a big slap shot and see it go in. If that happens today, a goalie is more likely to be chastised for letting in a "bad goal" rather than the shooter getting much credit for a great shot.
The advent of lighter graphite sticks has also contributed to the decrease of the slap shot. A hard wrist shot from a graphite stick can have as much or more speed than slap shots of old from the all-wooden sticks of yesteryear.