I agree that decaf coffee often has too much caffeine for someone who wants to practically eliminate it from their intake. Some decafs have as much as 7mg per 8oz cup, but there are some Swiss water process coffees with as less than 2mg. Don't take that as a recommendation, just information.
I don't drink any decaf coffee, but I will drink decaf tea or hot cocoa, either of which might have up to 2mg of caffeine in a cup. I also eat dark chocolate which has some. But I don't drink or eat any of those things habitually the way I drank coffee before. If I had a habit of drinking two cups of decaf a day, my monthly intake could be more than 420 mg (or 120 mg with swiss-water decaf). Non-habitual decaf tea or chocolate consumption results in something closer to 24 mg per month. I drank herbal teas habitually when I was changing my behavior away from a coffee consumption habit. I still drink them occasionally, but not on a daily basis like I did for a while. I do a lot of outdoor stuff and live in a cold climate so hot drinks are useful for staving-off hypothermia.
To anyone who consumes caffeine and other stimulants (guarana, taurine, and L-carnitine) habitually, I recommend quitting for a year and seeing what you notice about your mental and physical health and well-being. You can argue that's there's nothing wrong with it, or that it's healthy, or that there's no proof, or whatever. I don't care. It's not my gospel. On your part, you will certainly save money that you would otherwise spend on overpriced beverages with those chemicals that you don't actually need. What's more, you will prove to yourself through your own experience whether it makes any difference to you or not. Just don't quit for a couple of days or a month or two and think that you've figured it out. A year is a short time to go without those stimulants, but enough time away from them that you can make a sound decision about whether to return to them or not.