As another licensed title published by Bandai NAMCO, the likely cause for the delisting is a familiar but somewhat unexpected one in this case: expired licensing. Although Bandai owns the Tamagotchi property, creating and releasing games based on it through the Bandai NAMCO Games division likely still requires licensing. As the game is about five years old at this point the work to renew the release likely outweighs the potential sales, especially given the impending closure of the 3DS eShop.
At least four older 3DS Tamagotchi titles from Bandai NAMCO have previously been delisted including: Cho~ricchi! Tamagotchi no Puchi Puchi Omisecchi de Violin Lesson, Tamagotchi no Doki Doki Dream Omisecchi, Tamagotchi! Seishun no Dream School, and Guruguru Tamagotchi!.
Fortunately Atsumemashita, along with the majority of the titles listed above, did see a physical release although tracking them down nowadays might be a bit of a chore. Thanks to StarlacUK for pointing out the news and Wakayama_PT for reminding me with a submission. I’ve added the date to the Watch List calendar and will have a page up on the site later.