Just came across that template "expandable" variable now, was crafting a new template set for a custom department, and saw the help text:Ticket Thread expansions.originalOriginal Message.lastmessageLast Message
You'll note they are "Ticket Thread expansions", so, if you put %{ticket.thread.original} you'll get the original message, similarly %{ticket.thread.lastmessage} the last message.. but not the whole thread.