Yes, I believe that you are correct. That property (fullcalendar axisFormat) has disappeared. If I try to look at the GitHub repository, it appears as if it was last included for the commits of February 27, 2015 and had either intentionally or accidentally been removed in the commits of March 2, 2015. Apparently you are the first to have noticed that it is missing in action ...
Hopefully Roeland Salij or one of the other Mendix folks that is keeping an eye on this widget can restore this functionality. As I recall, setting of this property used to occur on the Extra panel under the name Agenda axis time format. As I think that most of the world uses 24-hour notation, I should think that this should be restored quickly. Because this is functionality that is fully supported by the underlying fullcalendar widget, I believe that it should be simple to add this back in.
It is probably best to go to the GitHub site for the Mendix Calendar widget and raise this as an issue.