Inbox was a testing ground of sorts for new Gmail features.
Many of these features slowly became redundant though, as both Gmail and Android itself started adopting Inbox's core features. The app which served as an experimental testbed for Gmail features as well as a streamlined email client for the service, has apparently outlived its welcome with the company. As a result, we're planning to focus exclusively on Gmail and say goodbye to Inbox by Gmail at the end of March 2019.
The page suggests alternatives for the Inbox features pin, bundles and reminders. This merging of features led to uncertainty about the future of Inbox, and ultimately, to its demise.
We introduced the new Gmail in April this year, incorporating numerous same features you've come to love about Inbox plus newer features like Smart Compose, which helps you draft emails faster. It's a service many of us use every single day. It was built around mobile devices and perhaps for the first time, it eschewed the "everything but the kitchen sink" mentality of most email apps at the time. In it, Google walks customers through using some of their favorite features from Inbox in Gmail as well as discover new ones.
Google Gmail product manager Matthew Izatt wrote in a blog post that Inbox users can consult an online guide to familiarize themselves with Gmail.
I know quite a few Gmail users who switched to Inbox when it was released.