I'm watching Stranger Things and the central plot point involves this entity, which I have heard of in other places. It seems to be a fairly common and recurring theme among sci-fi and dystopian stories, and even mainstream ones in games and other movies. Is there any evidence to back up such theories? If yes, why don't we know more about them? Did such experiments on humans actually happen? Did they yield anything? Do we know if other countries had similar programs? If no, why is it so common? Is this just like Roswell and Area 51?
In a word, yes. MKUltra was a totally real and well documented CIA program, though fictional depictions are, of course, fictional. (I haven't seen "Stranger Things" but doesn't it take place in the 1980s? MKULtra ended - or so the CIA would have use believe :P - in 1973.) Additionally, Roswell and Area 51 are real places. Hell, I've met one of the Lockheed test pilots who was sent to Area 51 to test the SR-71.
Humans absolutely were experimented on and the use of psychedelics by MKUltra possibly played a part in the popularization of those drugs, as at least two influential counter-culturalists (Grateful Dead lyricist Robert Hunter and Merry Prankster Ken Kesey) were first exposed to psychedelics as MKUltra test subjects. When reading about the far-fetched ideas the CIA had about psychedelics and hypnosis, it's important to keep in mind that MKUltra was very speculative and that it started in the early 1950s.