Shoutbox

It is still there but it is now to be found in the Quick Texts window. (Preferences > Conversations > Quick Texts)

Click add and the following window comes up:



Select the "Automatically replace text in the messages I send" radio box (as in screenshot) and fill in the boxes underneath.

Hope this helps

EDIT: as for replacing incoming text, this isnt possible with Plus! alone but can be done with the new scripting feature