Package: https://software.opensuse.org/package/plasma6-keyboard
You see that you don’t see package for Leap 16.0, but it is there:
Add repo:
https://download.opensuse.org/repositories/KDE:/Frameworks/openSUSE_Leap_16.0/
Then install plasma6-keyboard.
But it requires new Qt, namely, libQt6Core (Qt_6.10).
To get it use this: https://software.opensuse.org/package/libQt6Core6
Add repo:
https://download.opensuse.org/repositories/KDE:/Qt6/openSUSE_Leap_16.0/
After that you can install plasma6-keyboard. But this requires for Leap 16.0 too much package upgrades and some deletions (Dolphin & others). And it is better to upgrade the whole KDE 6 from default 6.4 to 6.6+ instead of such partial one.
ILL installing plasma6-keyboard in Leap 15.6 + KDE 5 is less harmful.
Maliit keyboard is available for Leap 16.0 from Main repo - try to use it before such big upgrades.