The appearance of the demo came without about as much fanfare as the announcement earlier this week that the Denuvo anti-tampering software had been removed from the game—which is to say, virtually none at all, aside from this tweet. But "low-key" seems to be how Playdead rolls. The full description of Inside is a single line stating, "Hunted and alone, a boy finds himself drawn into the center of a dark project." That is literally the whole thing.
Inside and its Playdead predecessor Limbo are on sale on Steam and GOG, but Inside is actually slightly cheaper on the Humble Store: $14, compared to $15 elsewhere. (Hey, a buck is a buck.) Limbo is currently going for $2/£1 on all three storefronts. The demo, however, is only available on Steam.