To disable the on-screen keyboard displayed by the SDDM display manager in KDE Plasma on Debian/Kubuntu/etc after installing SDDM themes, paste the following config into a new file at /etc/sddm.conf.d/disable-vkeyboard.conf (or any filename, in the same directory):
[General]
# Disable virtual keyboard at login
InputMethod=
Note: Some answers found by a quick web search say to edit /etc/sddm.conf directly, rather than /etc/sddm.conf.d/*.conf – if you edit this file, it’s likely to be overwritten later on by the Plasma desktop settings application, whereas “*.conf.d” directories are a standard practice for user configs anyways. So just use the sddm.conf.d directory.
Worth a try. The correct path would be /etc/sddm.conf.c/disable-vkeyboard.conf
EDIT: didn’t work. The onscreen keyboard appears anyway. It would already be helpfull if the keyboard sports all characters needed for a secure password. I have a ; in one and that is just four clicks away. I wonder who does useability checking @ kde development. it looks like this is left to the developers, unfortunately.