I don't think there is much doubt that the movie presented a supernatural phenomenon and not a mental breakdown.
Kubrick, being the true artist that he is and the greatest director of all time IMO, makes his films subjective and open to varying interpretations depending on who watches it, in this case, blurring the line between the supernatural and psychosis. Mike Flanagan, who directed Doctor Sleep
, which is also based on a Stephen King novel and is the sequel to The Shining
, did direct a film with supernatural elements, which in hindsight is making me wonder if Kubrick directed a supernatural thriller, disguised as a psychological thriller.
For example, the Dick Holland character who was clearly killed in Kubrick's masterpiece, reappeared as a ghost in Flanagan's sequel. The characters in the sequel also had supernatural powers and were immortal just so long as they continue to consume steam that children with the shining produce. Stephen King was disappointed that Kubrick heavily omitted the final events at the Overlook Hotel, so I'm now wondering if he omitted supernatural events from his adaptation, causing Flanagan to feel compelled to include these supernatural elements in his adaptation, in order to seamlessly blend the two Stephen King novels together in the film adaptations.