It seems that in reasonably current-ish KMail (5.11.3) the “Folders to monitor for new messages notifications” setting is not saved.
STEPS TO REPRODUCE
1. Go to KMail settings > Accounts > Incoming > “Other actions” button > “Folders” tab > “Select which folders to monitor for new message notifications”
2. By default, all folders are selected. Unselect any checkboxes whatsoever and press OK.
3. Go back to 1.
The setting is not saved.
The setting should be saved.
Linux/KDE Plasma: openSUSE Tumbleweed 20190810
(available in About System)
KDE Plasma Version: 5.16.4
KDE Frameworks Version: 5.60.0
Qt Version: 5.13.0
The bug must be fairly recent. I hadn’t encountered this in either Debian buster or openSUSE Leap 15.1.
Fixed in 5.12.0