It's been there for a long time. 🙃
I think some distros come by default with maliit keyboard, you just need to enable it, or install then enable. Personally I am using corekeyboard, it's the only one with modifier keys (somehow neither gnome nor kde default ones even allow to modifiers).
@nesc I tested both but both are neiher full sized keyboards nor work wit input methods like fcitx or have issues with key combintion and stuff like that.
I tested 12 OSK for both GNOME and Plasma and this here was the only one that was actually able to replace a full size keyboard in all circumstances