I installed krdc-16.03.90 and the new connection button did not work. The list of connections was also empty, despite having one before in the krdc-15.* versions. Rolling back to krdc-15.* allows me to continue using krdc.
Same for me on kde-frameworks/*5.20.0:5.
The "new connection" button switches to the "new connection" tab (which also happens to be what you see when launching krdc). I agree this is a bit unintuitive, but the behaviour for me is the same between 15.12.x and 16.04.x,