For shapes it is currently possible to adjust the transparency of the fill, the outline and the included text. However, to my knowledge there is still no option to adjust the transparency of the shape including fill, outline and text as such. This is despite the fact that this effect would be very useful to guide the audience from one element to another on different slides. Often this is achieved by semi transparent overlays and , but really that is just a mediocre workaround. Another, better, workaround is adding a transparency animation right from the start of a slide, but it makes designing rather cumbersome as it's not WYSIWYG.
That's why in my opinion a slider for "overall transparency" is needed for shapes and groups.