kde-plasma/systemsettings should depend from x11-apps/setxkbmap (like kde-base/systemsettings). Without setxkbmap is not possible to configure keyboard layouts. Reproducible: Always
Thanks, fixed in git. https://gitweb.gentoo.org/proj/kde.git/commit/?id=821ed7b14ea38ad2de2519e90d5f2321485eb2c7
It turned out that the keyboard KCM is provided by plasma-desktop.
Okay, thank you!