The labels on the vertical axis cannot be angled, at this time. There is usually not much need to do so for vertical axes, since the reason for angling labels on the horizontal axis is to allow more labels to be displayed along the width of the axis. But there would be a small benefit for vertical axes as well, though not to fit more labels but to allow the width of the labels to be reduced a small amount. That is not much of a benefit, however, and might not be worth the effort of making a change to support it.
Do you have another reason in mind for wanting to angle the labels on the vertical axis?