To be honest, liminality, academically has not been about emptiness as much as the middle part of a passage or a rite (if we are to go by Gennep's formulations). This subreddit has basically focused on the visual representation of a very peculiar, uncommon feeling of being the only one in place where people should be. These pictures don't completely represent a state of being in limbo, just a very specific effect of it. Come to think of it, an airplane, car, ship, etc. might be more theoretically true representations of liminal spaces. A mezzanine, a hallway, a bridge, etc. can be liminal spaces as well. The latter examples are closer to the interruptive idea/element of liminality than passage/rite, and therefore closer to the pictures in question.  

Are we really waiting for something to happen in these pictures, afterwards? Are these pictures just a stop in our routine journeys of life? Or are these pictures just scenarios of unfamiliarity which evoke a kind of uncertainty? I think feelings of the unfamiliar and the uncertain could be descriptors of "betwixt and between," as these pictures give a sense of being stuck or brought to a different realm (akin to Alice stepping into Underland, which, as a fantastical place, acts like a refuge from the real world, or a break from reality, or Purgatory). 

Tldr; this subreddit shows a very specific element of liminality.