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Similar topic: Accounts overview screen and K-9 Mail is back (5.800 release) .
TL;DR:
Existed up until 5.7xx Beta began and since been requested to return multiple times. Was originally planned for 6.200, but was postponed due to the urgent OAuth issue with Google.
When you opened an account from the overview, either a specified folder’s contents, or the folder list would display (depending on the auto-expand setting).