https://www.livescience.com/55479-does-caffeine-cause-dehydration.html
https://thetakeout.com/good-news-coffee-and-tea-are-apparently-not-dehydratin-1823893596

Source 1 talk about the idea that caffeine causes dehydration was first discourse in 192. They found people who increased their urination drank caffeinated drinks, and that caffeine was a diuretic. Professor Lawrence Armstrong is director of the laboratory for kinesiology and human performance at the university of Connecticut. The idea spread, and caffeine’s reputation as a dehydrating substance was cemented. According to a 2016 survey by the center for public health nutrition at the university of Washington. While the stimulant doesn’t cause dehydration, it also has some so-called side effects, including rumors that it could stunt human development or cause cancer.
Source 2 it also says caffeine doesn’t dehydrate people only increase urine. It may be some sort of ancient culinary legend that coffee and tea are dehydrating. Caffeine is indeed a mild diuretic, which means it causes people’s kidneys to excrete excess sodium and water through their urine. Dr. Daniel Virgil, an associate clinical professor of family medicine at UCLA’s David Geffen school of medicine, explains that it’s reasonable to think that people might be dehydrated if they urinate frequently, causing a lot of fluid to drain, but that’s not the case.
To conclude, Caffeine dehydrates people is a swindle. Caffeine in crease urine doesn’t take water away from human body. People think it dehydrates because when people pee they lose water from the body and they more they pee the easier they will get dehydrate which create an illusion of caffeine will dehydrate human body.