I know this was requested since Cubase 8 if not earlier. Last post I found was in September last year.
So it seems not only I feel the lack of this feature - note stretching inside key editor. It’s like we already have the stretching of controller values when we select a bunch of values in controller lane and click&move mouse near the bottom of lane. It is brilliant feature. So the same would be nice to have on note events - when we select notes then imagine the range covering all selected notes and you have a stretch line for clicking and dragging.
The difference between controller values and notes is that notes have “on” and “off” events. So I think by default this stretching should affect any event in selected region (selected notes), so the boundaries thus not movable should be the “on” event of first selected note and “off” event of last selected note.