This is pretty self explanatory. I see some members of the VDG push to the sddm-kcm module that's why I post it here.
Actually it's more about UX than design, so feel free to move this to a better suited section please.
i bet thousands would appreciate a simple check box to enabled/disable the Qt virtual keyboard from the sddm-kcm "Advanced" tab.

I lately wrote a few very well received themes for SDDM and although I now adapted them to work with the virtual keyboard you can see on the SDDM issue tracker that it's a very common issue. The checkbox would go in the "Advanced" tab and set "InputMethod" to either empty or to "qtvirtualkeyboard". Should be pretty straight forward for anybody maintaining the module.
A check for the corresponding package could be included to hide the option when the package is not installed at all.

I made something real quick: https://phabricator.kde.org/D22627