There was a bit of a push when these plans were announced to alter exactly where the new underpass would connect with Campbell arcade - while the large shop directly to the right of the ticket barriers was derelict and had been modified relatively recently, the smaller shops further along the arcade under Flinders St were pretty much in their original state and had tenants. The proposed entrance would have removed all the shops on the right hand side (including the barber at the corner next to the stairs to Flinders St, who's been there for a number of decades), and put the shops on the left behind the ticket barriers, making them a lot less attractive to the kinds of business they currently have.
Going by those plans linked to above, that hasn't changed - but it does explain why those plans are "subject to a future heritage permit", which I assume has been sorted out one way or another by now.
I believe the space behind the telephone signs was recently (a year or so ago) used for a hipster bar, and my guess is that once construction is complete they'll be looking to put something similar back there.
*edit: no, it seems like the Heritage Permit side of things as far as Campbell Arcade goes is yet to be sorted: https://www.theage.com.au/culture/art-and-design/sticky-has-reopened-its-doors-but-be-careful-with-the-zines-20200611-p551nv.html