The K-9 5.600 version started with the overview of all mailboxes
From that overview I could easily jump into specific mailboxes. I know, this overview is available in 5.800, too, but requires mores clicks to go there and it is less comfortable.
The workaround is (and supposedly the intention was) to use the shared inbox instead. The shared inbox adds a little colored rectangle to each mail to indicate the responding mailbox. I know it is hard to understand for non-colorblind people: But colors are no reliable information to me, even if I select colors for the various mailboxes I can distinguish, I cannot remember “blue is this mailbox and red is the other one”.
In case of deleting mails, it would be helpful if the marked mail would get a black bubble instead of a blue one.