After updating Kmail to 5.8.0, it is no longer possible to hide the "HTML side bar". In KMail 5.7.3 there is a checkbox (Show HTML side bar) in the Setting dialog, in Appearance -> General. Maybe this is related to the new MessageViewer::MessageViewerSettings::self()-> mimeTreeMode2.
*** This bug has been marked as a duplicate of bug 393421 ***
The linked bug was closed without a valid reason. Reopening this one to have a place to track the missing functionality