The delete message action no longer works with the SHIFT-DEL keyboard shortcut. Setting the shortcut to something else (I tested with SHIFT-P) or adding an delete icon in the toolbar does work.
This is in Version 5.4.0 Using :
KDE Frameworks 5.29.0
Qt 5.7.1 (built against 5.7.1)
The xcb windowing system
Expected behaviour :
When viewing the message list, pressing SHIFT-DEL should delete the selected messages.
Current behaviour :
In the message list, when one or more messages are selected pressing SHIFT-DEL does nothing.
I think I'm experiencing the same issue. Specifically, changing the selection in the message list sets focus to the message preview pane. There, Shift-Del does not work (but e.g. Del alone for "move to trash" works). On the message list itself, Shift+Del does work (if you manage to get focus there).
I don't know which of the two (focus change or no shift+del on preview pane) is new, but it's quite aggravating in combination.
Version: kmail-16.08.3 on Fedora 25
I also suffer from this bug. I can delete only once after the KMail started but then nothing happens. If I change the shortcut to something else I can delete once more again but then it doesn't work again.
It's also working when I select the message again, so it could be a focus issue.
It's very annoying as I used to use Shift+Del since KMail 1.x era.
Currently I'm using KMail 5.5.3.