And it looks like the built-in arrow marker type "may" (their word not mine) now be referenced by shape://arrow. This being the new way to symbolize the marker as either an arrow (shape://arrow) or an ellipse (shape://ellipse), replacing MarkersSymbolizermarker-type.
Looks like this is a Mapnik reference.
CartoCSS is actually converted into an XML format that Mapnik uses to render the stylings of the map.
And it looks like the built-in arrow marker type "may" (their word not mine) now be referenced by
shape://arrow. This being the new way to symbolize the marker as either anarrow(shape://arrow) or anellipse(shape://ellipse), replacingMarkersSymbolizermarker-type.