av.format.ContainerFormat object has no attribute 'variable_fps' in av>=14 #1741

After opening an mp4 file with container = av.open(filename)

accessing container.format.variable_fps returned a boolean with av version <14

With av version 14.* this line fails with

AttributeError: 'av.format.ContainerFormat' object has no attribute 'variable_fps'

Is this expected behavior? If so, it it documented somewhere and is there a suggested alternative to get the same functionality?