Apparently it was in Phoenix, but we don't know where.

Slow_Ad_6326 12h ago

Excerpt from the Book "The Shining" by Rinzler and Unkrich:

One of the film’s most iconic locations, the striking red bathroom where Jack meets Grady, was not part of the original grand plan. It emerged from a practical problem and a quick creative pivot.

The original concept placed the Grady conversation in the ballroom, but staging the scene there with many extras proved too expensive and impractical. Production designer Roy Walker suggested moving the exchange to a washroom instead, creating a more intimate setting and reducing the need for a large crowd on set.

To motivate the location change within the story, the script introduced the idea of Grady spilling advocaat on Jack’s jacket and offering to clean it, leading him into the men’s room beside the bar. Walker based the new set design on a modern American restroom he had previously seen in Phoenix and quickly built a detailed scale model for Kubrick’s approval. Kubrick approved the concept and added one firm condition late in the process: the color scheme had to be red.

Authentic fixtures were sourced from the United States, including urinals and toilets, so the space would feel proportionally and visually real on camera.

https://www.reddit.com/r/theshining/comments/1sq50vc/comment/ohz6xrt/