It can be but ther are simple and effective solutions. Wet timber isn’t terribly slippery (think about wooden piers, boat decks and so forth). What can happen however, is that if the deck is shaded and receives very little sun, it can get an algal or fungal growth on the surface, especially in the winter months. It is this algae which is slippery when wet and it applies to any surface, not just timber. So the easy solution is simply to keep the deck clean. There are proprietary products available but in my experience, a pressure washer works well and does no harm. Decking can be very slippery when it has a frost on it though. Personally, what works for me is a sprinkling of rock salt on the decking steps at the front of my house. An expensive but effective remedy is to fit non-slip pads or strips which fit in the decking grooves.