House Of Leaves is similiar to Backrooms in a way - they represent the same kinds of horror but it's more like how Weird Fiction converged and inspired in the same way. There's a level of early slenderman there as well (in terms of a lot of the early slenderman horror being more about the horror of dreaming this entity into the real)
I'd argue SCP Foundation is probably one of the main initial examples of internet occult, the Backrooms have more in common with a few SCPs.
To go wider there's probably a convergent horror - It's the classic aspect of horror stories of the age represent subliminal fears of the age (e.g, bloodsucking vampires mean very different things across the past thousand years). I think liminal horror is a representation of a lot of our fears, so multiple different effective horrors have converged on these feelings of discomfort with spaces.
I'm not really smart enough to know what it means - probably something about modern society dissassociating people from the space, but don't know much more.
I think it has something to do with the controlled comfort of modern life. And how even a small disruption can become unsettling.
Like in HoL, the most chilling scene isn't anything that happens beyond the door imo. It's when the book falls because the house changed size ever so slightly. I think the classic haunted house trope is at play there too - home is comfort but those stories frequently involve a move, which is inherently stressful. I remember when I bought my house..every new noise or small change was disturbing. Potentially a hidden horror lurking in the house (like a water leak).
edit: Another thing I will say is that I've noticed both HoL and Backrooms seem to act like a kind of shibboleth for a particular demographic (not even really the same demographic) and you often see this in how people write/talk about both. I think it maybe stems from how dense/unapproachable the two works are, how innocuous they seem on the surface such that you really have to sink some effort to get at them.
I don't have any real proof for this, but it feels like House of Leaves inspired a lot of the people making "found footage" and "creepypasta" stuff one the internet in the 2000s and early 2010s (SCP, Marble Hornets, Slender Man), and then that stuff came together to inspire the Backrooms.
Ironically enough, I first read this book when I worked at Disneyland, at an attraction with tunnel access, that no longer exists, and I no longer have the book because I loaned it to a girl I worked with there who never gave it back. She and I hung out in those tunnels during breaks.
Also interesting to me seeing other responses about found footage, given the guy who first recommended this book to me was a friend from college who covered his dorm room in bizarre free-form prose that switched between play style and novel style during a manic episode, then tore it all down later, set it on fire, and pieced together what survived as the strangest pseudo-poetry that somehow still had a lot of real words in it, kind of nature's "found poem." This book was probably written around the same time Blair Witch Project first came out and this guy also lived next to the real Greenway Trail in Burkittsville, which was also not particularly scary to see in person. I kind of doubt that was really the first example of a found footage film but it's the first I'm aware of. Wikipedia is saying The Connection from 1961 is the first.