In the v1.1.9 theme update we updated from Font Awesome v3 to v4.2. You’ll need to use the new icon class names now, they all changed when Font Awesome went from v3 to v4.
The Font Awesome icons are here → http://fortawesome.github.io/Font-Awesome/icons/
Some example of how to use them are here → http://fortawesome.github.io/Font-Awesome/examples/
For the example you posted, the new class names for that would be this…
<i class="fa fa-plus fa-lg"></i>
(Incidentally, that is not a shortcode, that is HTML.)