The Qt Virtual Keyboard [1] isn't exposed anywhere except the login screen. There are no options anywhere to find it in the accessibility settings. I have not confirmed if this is different in the wayland session [2]. For users who cannot use a keyboard (this can be for various reasons such as: no keyboard, defect keyboard, having a mobility impairment) this is a big usability issue.
[1] https://packages.ubuntu.com/source/bionic/qtvirtualkeyboard-opensource-src
[2] https://blog.martin-graesslin.com/blog/2016/05/virtual-keyboard-support-in-kwinwayland-5-7/#comment-70808