@funshark if you are using Liner curve, you will not have handles. Let's say you have 3 keys, A,B and C. you want the interpolation between A and B to be linear and between B and C to be Bezier, the B key will have only one handle on the right side of the key, like in your example.
If you put key A to be Bezier, key B will have handles from both sides as the interpolation between keys A and B is now Bezier.

Keep in mind if you want a key to be linear, when you select the key and apply the Linear curve, the interpolation between the selected key and the next one will be linear, hence no handles.

Fantastic work on the graph editor, a much needed feature for workflow.

+1 agree with @funshark, it's a bit of a different workflow to work with keys this way. Unity and many 3d software graph editors work by selecting a keyframe and being able to edit the bezier handles on the left/right of the selected keyframe, and not the following unselected keyframe. Not the end of the world, but maybe that way because of the way the current code base is setup?

+1 also wondering if there's any rough ETA for v4 release? I'm a little nervous to introduce a beta version in production, but would really like to use this feature.

@funshark, I'm not seeing that behavior in 4.0.75-beta. What version are you using? For example with this curve and selected key:
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After pressing the "flat" preset button it becomes:
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The preset buttons should only affect the handles that originate from the selected keys.