Nintendo's first mobile app, Miitomo is officially dead

We want to thank you for enjoying Miitomo so much. Miitomo’s service has come to an end as of Wednesday 9th May 2018, 16:00 (JST). We’ve come a long way since the app was launched, and once again we want to thank everyone who’s been involved. We hope to go on to even bigger and better things in the future with Nintendo, and we hope you’re there to share the fun. We hope to see you again soon!
Nintendo also announced that while Mii characters can be transferred to Nintendo Accounts by linking it to Miitomo, their personality and other info will not be saved.
source: Nintendo