It's no longer there. I've been told on IRC that you can achieve the same thing using the Composite transition, adding a keyframe at the end of the transition and setting its opacity to 0, but it would be great if someone could explain this in detail. Which of the two clips involved does opacity refer to? What is the 'Distort' button supposed to do? I see some strange artefacts with the transition placement and size because my source clips are a non-standard size.

OK, I got this working according to those instructions - but I had to set resize to 200% for the keyframe at the beginning of the transition to remove the resize artefacts. No idea why, have submitted a bug:

http://www.kdenlive.org/mantis/view.php?id=243