Plasma has a virtual keyboard, which should be activated when a text input field is selected and no keyboard is available. However, there are times you may want it even when a physical keyboard is detected as being available. So having a plasma widget that when pressed will pull up the virtual keyboard, like the button SDDM has, would be very useful.
Unfortunately there turns out to be no way to make the virtual keyboard work tolerably on X11. On Wayland, we can make the virtual keyboard appear only when needed: when you actually touch a text input field. We can also make the UI adjust so that the text field remains visible when the keyboard appears. None of these things would be possible for a manually-activated virtual keyboard. So I think we will have to stay with the current approach, sorry. :)