I took that out in version 4. Too many people were confused by it and kept saving images with a blue background and asking why the images were not extracted. Also, the new Mask Checker plugin is a much better way to check the transparency. With that in mind, I figure I would just drop this feature.
The Mask Checker plugin can show any color in the background without adding any layers. So when the file is saved, it will still have transparency and the user doesn’t need to remember to disable any layers.
The Mask Checker plugin isn’t my plugin. It is on the Adobe Exchange and is a one time purchase of $11. I use this plugin all the time for so many things. This is what I recommend using in place of the old enable background layer feature.