In case you are among the short list of people around the world that were using Inbox by Gmail. You might have found trouble using the mobile app or web version of the Google email client starting April 2. That is because Google shut down the conventional Gmail alternative.
Inbox by Gmail was designed with the hope it will replace the normal (traditional) Gmail. As it worked out, you were supposed to see important emails fast and first, and decide whether to reply then or later. You, therefore, had a snooze feature.
However, with time, Google started porting features from Inbox by Gmail. Bringing them onto the normal Gmail platform. It reached a point where there was duplicity of features across Inbox by Gmail and the normal Gmail. Coupled with the fact that the former was not all that popular, its shut down does not come as a surprise to many.
Nonetheless, when Google finally shut it down, it still has a considerable number of fans. These fans found workarounds to still keep using the app. The workaround entailed downloading older version APK of Inbox by Gmail, but that could only go for so long, as Google later came in and totally disabled any version of the app from working.
Right now, if you install older APK, you keep getting an error message during signing into your Gmail account. However, there is a Reddit thread that seems to suggest that users who had the older version and all along had automatic updates turned off are still able to use the older Inbox by Gmail app. Though they cannot receive notifications for new emails and refresh now has to be manual.