Long before Undertale made its debut, indie developer Toby Fox was already working on a successor. Last October Fox released the first chapter of this new game, Deltarune, as a free download on PC. Fox is currently attempting to recruit a team to help him develop the rest of the game, but in the meantime, that first chapter has already been ported to Nintendo Switch and PlayStation 4.
Unfortunately, it seems there's currently an issue preventing it from staying on the Switch eShop. The game isn't showing up for North American users currently, but it's still available in both Europe and Japan. It's unclear why only North America is impacted, but some are speculating it could be related to an issue with the game's ESRB rating. So far there's no official word from Nintendo or Fox, but we'll be on the lookout. Currently (10:15 PM Eastern) it is still available for download on the PlayStation Store.
Update: Toby Fox has now confirmed that Deltarune was pulled from the North American Nintendo Switch eShop due to "issues with the rating in certain territories." A solution is in the works, but it's unclear how long it will take. Hopefully the issue can be resolved soon.