In Deltarune, they live in the Lake next to Hometown, and can be visited by Kris in the first two chapters.
In Chapter 5, the Normal NPC is found at the Festival selling "fried dough ball[s] made of Octopus" which are also noted to taste like onion. That Chapter's Weird Route involves the Lake in a way that Onionsan could intervene to prevent, so soon after the chapter's release, it became a theory/joke that Normal NPC killed Onionsan and turned them into takoyaki as to allow the Weird Route to progress, with the words Normal NPC repeats to the player in Chapter 4 implied to be Onionsan's last.
