OSK's are so important for convertibles, tablets and phones, and in a pretty rough state on KDE and GNOME too.
Gnome has a nice one but very underpowered. And a GNOME extension that made it better is now broken, now sure if there is a new one.
KDE has maliit which is Qt5 and abandoned, and replaced it with plasma keyboard (which is also the name of the IME doing the fancy long-press-special-character-suggestions thing) but that was not well sized last time I tried it