There is no process that entirely removes it, it just greatly reduces it. I had no idea about this until I read the article on X. Someone shared it and I was like, what, NO WAY! It appears to be true though. I will link the article below the quote that explains it:
An eight-ounce cup of regular coffee typically has around 95 to 200 milligrams of caffeine, according to the U.S. National Library of Medicine. Decaf coffee typically only has between two and 15 milligrams per 8-ounce cup, according to the FDA. As with regular coffee, there’s clearly a lot of variability here. “How much caffeine is in there by the time it gets to the consumer mostly depends on how much caffeine was present to start with,” which varies with different types of beans and growth regions, Ristenpart says. But it also depends on the technique and intensiveness of the decaffeination process a manufacturer uses, as well as the brewing method and strength, he adds.
I found out about this years ago. I have a friend who loves coffee flavored items but can't have caffeine. She is allergic to it! She tried drinking decaff but was still getting mild reactions and her doctor told her that decaff just means some or most of the caffeine is removed, not all of it.
