then held down the option key
With the Scale tool selected as well as an image selected, It should be an Option click (and release) to open the scale dialog box. Double-clicking on the Scale tool in the toolbox does the same thing.
You still can do that. You need to select the Scale tool as well as the image/object and then hold the Option/Alt Key and click anywhere on the artboard to get the window you are talking about, so select percentage.