On Polygon, tabletop game historian, Jon Peterson (Playing at the World, The Elusive Shift) looks at the "Satanic panic" of the 1980s, which serves as one of the themes in season 4 of Stranger Things.
It is an interesting piece about how TSR certainly flirted with real-world occultism and demonology as they knew interest in the subject and its transgressive nature would attract players.
To those who think of Demogorgon as no more real than Thanos, the reuse of existing mythology in the Monster Manual may seem as harmless as appropriating Tolkien's orcs. But the co-creators of D&D, Gary Gygax and Dave Arneson, were both quite serious about Christianity. When people started questioning the use of "real-world" religion in D&D, Gygax drew the line at including angels, writing in Dragon magazine #43, "While little objection can be made to the utilization and slaying of demons and devils, who would dare say the same of angels? Surely you can recognize that game use of such is absolutely out of the question for those of the Judeo-Christian faith."
So for Gygax, at least, it was all right to include "real" demons in D&D as long as they were villains. But not everyone who played the game treated them that way. Plenty of early D&D adopters in the 1970s had a parallel interest in the occult. Some of that was the lingering hippie New Age vogue for astrology, crystals, and tarot cards. But other players weren't reading horoscopes so much as Aleister Crowley — they thought the occult was cool. Those sorts of people had done a very different sort of historical research, and they took D&D to places that Gygax would never have anticipated.