Decaf still contains some caffeine, about 10-20% the amount of regular coffee.
I recently started a new stimulant med and decided to stop consuming sugar, salty foods and caffeine. I tried to do it all at once and had two days of absolutely miserable headaches. Now I'm just on a much lower amount of caffeine, sugar and salty foods than normal, and I've been doing fine. It's good to taper things off over time.