The Publish button was always available, whether or not there was anything outstanding. Opening a published form you had not touched looked exactly the same as opening one with unsaved work in it, so the only way to be sure respondents were seeing your latest version was to publish again and hope.
It is now greyed out when the form you are editing matches the published one. Making any change enables it, and publishing greys it out again. So the button doubles as an answer to "is what I am looking at what people are filling in?"
Changing a form's appearance does not enable it, because appearance reaches the live form immediately and there is nothing left to publish.