gnome-shell has no support for caribou since 3.28 and uses ibus-gtk3 for Screen Keyboard. We may consider dropping
libcaribou*. We must check support in Tor Browser and GTK2 applications.
Stop installing caribou and libcaribou*: they're not used by GNOME Shell in Buster anymore (refs: #16628)
After removing caribou-related packages from
libcaribou-gtk3-module is installed anyway, and then the on-screen keyboard still works fine in GEdit, Tor Browser, and OnionShare. But it doesn't work in Thunderbird (no AppArmor denial in the logs). But actually, it doesn't work in Thunderbird at d7747a0555fea7af2a0c4ba2604d8a3f63c3f315 either, with all the caribou packages installed, so removing packages did not cause the problem => filed #16756 about it. Let's fix that problem before removing stuff.