This question was about an older Ubuntu version, but in case someone is looking for an answer for Ubuntu 18.04 or the current development version Cosmic 18.10, this may help:
The new Gnome on-screen keyboard in Ubuntu 18.04 is this one:
However, note that the description in these release notes is quite misleading (and the description in the Ubuntu help is as well in a different way: It depends on whether you use the default X session or Wayland, and enabling it in Universal Access settings may or may not help).
Following is a (slightly edited for correctness, though there may still be mistakes based on confusion) copy of my recent comment on an earlier bug report. You can find further links including additional new bug reports there: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1760399
Auto-summon of the new OSK in Ubuntu 18.04 only works with application/toolkit support. Text widgets in alien (non-Gnome or non-Gtk?) toolkits don't summon the OSK (Chrome, Firefox, Qt, whatever other toolkits; I don't know if this is even standardized by freedesktop.org.
Because of alien toolkits there is always a need to summon on demand. Since Gnome 3.28 in Ubuntu 18.04, you can summon on a touch screen by swiping up from the bottom of the screen. However, this ONLY works in a Wayland session, not in X. I can confirm that it does. None of this is described in the Gnome 3.28 release notes or the Ubuntu 18.04 help. These are clearly documentation bugs.
Judging by the confusion in this bug report and the many questions on Askubuntu,com, it seems that the swiping up eluded even those users who have access to the feature because they use Wayland. Quite clearly this is not sufficiently discoverable, which IMHO is a bug. (Of course this is not helped by the buggy docs)
The Gnome extension to summon on-demand (https://extensions.gnome.org/extension/1061/on-screen-keyboard-button/) does NOT work with touch in Ubuntu 18.04 (Gnome 3.28) according to the author, and neither does it in Cosmic (3.30). It works with the mouse but this is awkward to say the least. The author will hopefully be able to look into it but it may take some time.
I don't know why for me in Gnome 3.30 (Cosmic) the auto-summon only works for Activities and Applications menus search boxes, and not elsewhere in Gnome as far as I can find. Hard to believe that this is a bug in every Gnome application, maybe a Gtk bug?