In the previous layout, boards were listed in the left side-bar. From there one could deactivate a board.
Not possible anymore, not from this layout.
[Edit} just learned from Google AI: waaaaaay at the bottom of the board menu, I can close a board. In the board overview, I can select the closed boards and delete it from there.
All this would not be a problem if Trello incorporated a simple Delete option for cards, lists and boards. Now I have to jump through all kinds of hoops in order to keep the listcount of a board under the max of 4000.