I'm going to play devils' advocate here. I'm not sure I believe my own argument, but it's one that I think is important to, at minimum, meaningfully represent.
Literature was given the broadest possible scope at the outset, and it's our job to discover its boundaries. The default Tour really captures this:
Literature Stack Exchange is a question and answer site for scholars and enthusiasts of literature.
The problem is, literary questions arise in many, many mediums beyond just books. "Literature" as a field has never been limited to just books - movies, video games, you-name-it have all been able to fall under literary critique.
A question about how Oxenfree constructs its horror is totally a valid literary question, because the content being analyzed presents a narrative that's intended to be meaningful. A question about how Fight Club (the movie) looks at self-imposed harm even in the face of great efforts to escape it would still be on-topic here, because it's a question about how elements of a story come together to present a point or argument, either visual or verbal.
I'm reluctant to cut off any media as "not literature" when, quite clearly, there's much more to the presentation of meaningful cultural discourse than books.
So, maybe in that sense, the best way to limit this might be broadly (and obviously, this would take some refinement) by whether it tries to present a narrative - whether it has a text at all. If it does, then maybe we shouldn't be so quick to remove it from the site.
I'm not saying we have to universally allow everything that has a text, and obviously it would take some refinement, but I think we'd be wrong, and it would be limiting, to preclude other forms of media that present equally valid and meaningful narratives and points.