That tunnel was closed to implement a steam line through it. That work was completed. They are doing a continual program of work on those buildings with recapitalization work, especially on the Confederation Building. PSPC has been using some of that space for a construction yard. Some of the area around the Confederation Building itself is not safe because of the work that's going on above, so that's why it hasn't been reopened. If that work gets completed, then there should be no reason not to open the lawn.
On December 11th, 2018. See this statement in context.