OSK needs (at least the option of) modifiers (Ctrl/Alt/Super)
There are plenty of needs for modifier keys, that don't magically go away if the user isn't using a physical keyboard.
- Ctrl in the terminal, for many many things
- Alt-F2 for Shell's own run dialogue
- Alt to show Firefox's menu bar
There will be hundreds of examples like these where there is no UI alternative to a keyboard shortcut with a modifier.
The non-goal from https://wiki.gnome.org/Design/OS/ScreenKeyboard of "Not intended to support every possible key - modifier keys like ctrl/alt/super/F keys" is mystifyingly unexplained - especially since lower down on that page it says:
"On-screen keyboards that focus on accessibility tend to ... allow access to modifier keys like Ctrl, Shift and Alt. This allows any kind of keyboard input, without the need for a physical keyboard. This is probably because some applications require modifier keys for some actions."
At the very least, there should be the option for modifier keys.
In my case, I have a tablet with a detachable keyboard. Not having these keys prevents using it under GNOME without the physical keyboard attached, especially in the case of terminal use, where having no Ctrl makes it useless.