Everyone suspected it for a while. Their active Twitter account refused for months to say when the app would be updated for the iPhone X. Personally, I suspect that they painted themselves into a bit of a corner with the tech stack they choose for Inbox. They used a unique C++ to JS compiler to run Inbox in the browser. This worked decently well in Chrome, but the experience in other browsers has been a lot choppier.…
When you have a successful product people love, you don't sunset it because the tech stack is too cumbersome. This has to mean that the product wasn't seeing the adoption they were hoping for.
(BTW the same goes for the old HN hobby-horse, Reader. If it had had a billion users, it would never have gotten shut down.)