Undertale’s metafiction isn’t wholly new, but conflating metafiction with sentience is certainly, well, something. That is to say, Undertale speaks to, and judges a player for, progressing through its narrative. Not diegetically as some kind of commentary on the form, but from a point of object permanence. Stretched literary values assert Undertale as a living, real place. Therefore it treats a player as a literal infiltrator of Undertale’s peace.
Layers deep into this conceit the game comes to muster and essentially asks: why play this game, why do these things? I see this a question of taste, of aesthetics; altogether something that has more to do with my life situation. Of course, no character cares about why I literally bought, installed, and played Undertale. They just want me to leave them alone. Making this play a question of ethics, because of the asserted agency and reality of the videogame at hand, makes the metafictional implication of asking like, any possible meaning of finishing the game outside of its conceit basically subtext. Because as far as Undertale is concerned, its awareness is a natural structure.
Basically the post-undertale game is like “The Monster at the End of this Book.” Myself, the reader, share in the static reality of this book with Grover. He’s allowed to turn the page, change the contents of the book, because he exists of it and inside it. Though the “change” is relative to reading it. The page he nails shut is always nailed shut, and so on. Grover pleads in vain for the reader not to turn any pages out of fear for the monster at the end of the book. Spoilers for an iconic classic of literature: the monster is him!