This is a byproduct of us cleaning up the toolbar to make the app less overwhelming (and simplify things a little bit). While the button is gone, you can still show/hide markup with CTRL/CMD + K or via the View menu.
We’re hoping that once folks get used to this change, it will be even quicker for advanced users to wrangle their markup. However, if it continues to be a burden, let us know, and we will see if we can take another shot at it.