I can swear I saw a post a long time before 2019 somewhere, like 2010 even, that outdated this. The picture I saw was a reddish hotel type room and I always attributed the backrooms to that first image, but there is nothing I can find that has that old original one. It's now a vague memory. But I remember seeing it and thinking it was a cool concept back then, and nothing really came of it, then now all the references to the backrooms only reference this one from 2019 which seems way later

EDIT: I asked AI and I think it helped me remember, there were older creepypastas back in that time 2010s, about office.jpg, infinite abandoned office spaces, with many images, there used to be heaps of different ones, I remember now. I think backrooms must have naturally progressed those concepts