Google has officially shut down Inbox by Gmail on April 2, 2019 and transition all its users to Gmail App. While a lot of features have been migrated to Gmail, some users still miss features like bundle for Gmail.
A recent report also suggest Google might be damaging its brand image by often shut down of some services / platforms.
Good thing is we can still download and install an old version of APK from other source, download link here. It is signed by Google and verified so feel safe to install it as 3rd party source.
After that make sure to visit Play Store > hamburger menu > My Apps & Games > Installed > find and tap “Inbox by Gmail” > uncheck “Enable auto update”
For iOS users, the official app still works at the time of this writing. But it will eventually be taken down, and Google seemingly hasn’t terminated backend yet, so enjoy it while you can. Unfortunately there is no good way to get the App outside of AppStore since Apple does not allow 3rd party install.