I tried OKBoard. It worked.
Then I switched to FUTO, which also worked.
Then I uninstalled OKBoard.
Now I have no keyboard at all, even when I select one of SfOS’ default layouts.
I rebooted.
I uninstalled FUTO.
I rebooted.
No keyboard.
The solution is here. If ~/.config/maliit.org/server.conf only refers to OKboard and nothing else, it is safe to delete it completely.
And Utilities now features a Restart Keyboard button.
OP can ignore this. It only affects people who previously used OKBoard.
edit: and of course I reinstalled FUTO keyboard.