Although it has grown to become an excellent Gmail alternative, Google's Inbox app remains an experimental piece of code. This means that Google is using the data provided by its usage to improve Gmail and, unfortunately, it seems that the internet giant has decided to discontinue Inbox at the end of March 2019.
https://www.notebookcheck.net/Google-plans-to-shut-down-the-Inbox-app-by-March-2019.331472.0.html