Drinking too much decaffeinated coffee can be bad for you.
From what I've heard years ago decaffeinated coffee is supposedly harmful to your heart health and can have harmful effects on the heart by increasing the levels of a specific cholesterol in the blood.
Caffeinated coffee is the much healthier option and may actually improve your heart health but drinking decaffeinated coffee in moderation should not do you any harm.
Sometimes I like to drink coffee in the evening and I'll drink the decaffeinated coffee instead of my regular caffeinated coffee so that it won't keep me awake at night.
So drinking decaffeinated coffee in moderation is not harmful although drinking too much decaffeinated coffee and only decaffeinated coffee can eventually lead to heart problems.